Rutile Surface Reactivity Provides Insight into the Structure-Directing

Article pubs.acs.org/JPCC

Rutile Surface Reactivity Provides Insight into the Structure-Directing Role of Peroxide in TiO2 Polymorph Control Anqi Song, Dapeng Jing, and Melissa A. Hines* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States S Supporting Information *

ABSTRACT: The peroxo ligand is one of the most promising structure-directing agents in TiO2 nanocatalyst synthesis; however, its role in nanocatalyst growth and polymorph control is not well understood. Using a combination of scanning tunneling microscopy, X-ray photoemission spectroscopy, and kinetic Monte Carlo simulation, we show that the base-catalyzed reaction of H2O2 targets very particular sites on the rutile (110) surface, sites that are present in concentrations of less than 1% of a monolayer, producing highly homogeneous surfaces characterized by flat terraces and straight atomic-height steps. This reaction produces surface steps with a different orientation, different structure, and different reactivity from those prepared in ultrahigh vacuum studies. The observed reactivity is explained by a simple, site-specific model that is based on metal oxo coordination chemistry. This study shows that one of the principle roles of peroxide in etching and growth reactions is destabilizing neighboring bonds and increasing their lability. As a result, the peroxo ligand adds a degree of reversibility to growth reactions, thereby promoting the formation of well ordered crystals.

1. INTRODUCTION Nanoscale TiO2 has received a great deal of attention over the past decade due to a number of high-profile applications, including dye-sensitized solar cells1−4 and photoactivated selfcleaning or environmentally remediating surfaces.5−8 In these applications, the reactivity of the catalyst is sensitive to both surface structure and crystal polymorph (bulk structure). In the case of TiO2, three polymorphs are stable at atmospheric pressure: the thermodynamically favored rutile phase which can be grown in bulk and the less common anatase and brookite phases. The polymorphs have significantly different chemical reactivities with anatase and brookite typically being thought the most reactive, so many researchers have pursued polymorph-specific syntheses of high reactivity nanocrystals.9−12 To date, these pursuits have primarily been empirical, as crystal polymorph is determined in the very earliest stages of nucleation and growth, a regime that remains impossible to image in spite of recent impressive advances.13 The peroxide ligand, O22−, is one of the most promising structure-directing agents in TiO2 nanocatalyst synthesis.14 For example, brookite-specific growth has been demonstrated from peroxide-containing molecular precursors,15 and polymorph selectivity can be tuned with peroxide in sol−gel syntheses.16−19 The allure of peroxide is based on two principal ideas. First and most obviously, the peroxide ligand contains no unwanted atoms, such as C, that can contaminate growing crystals. The second factor is kinetics. Aqueous titanium chemistry is dominated by extremely fast condensation reactions, so most approaches to controlled growth have © 2014 American Chemical Society

incorporated low-reactivity ligands in molecular precursors. Inspired by the unusual stability and “soft” nature of Ti(IV) peroxo compounds,20 peroxide-containing precursors have found wide application in the synthesis of low-dimensionality nanoscale metal oxides.14 Prevailing wisdom holds that the bidentate peroxo ligand blocks condensation reactions, thereby stabilizing and directing nanoscale growth.14 In this work, we show that peroxide plays a third role in nanoscale titanium chemistry. The peroxo ligand destabilizes neighboring bonds and increases their lability. On rutile surfaces, this destabilization leads to highly site-specific etching. During growth, the peroxo ligands will do more than just slow down condensation; they will add a degree of reversibility to the reaction, favoring the formation of well ordered crystals over amorphous materials. We further show that peroxo ligands lead to the formation and stabilization of an unusual step structure on rutile surfaces, a structure that is entirely different from that produced by standard ultrahigh-vacuum (UHV) surface science techniques. As a result, TiO2 surfaces prepared using standard UHV surface science techniques may not accurately model real-world catalysts. As is typical for growth and etching, the reactions studied here occur at very special sites on the surface, sites that are present in concentrations of less than 1% of a monolayer. This density is below the detection limit for most surface Received: July 21, 2014 Revised: October 3, 2014 Published: October 30, 2014 27343

dx.doi.org/10.1021/jp507292v | J. Phys. Chem. C 2014, 118, 27343−27352

The Journal of Physical Chemistry C

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atoms to adopt their thermodynamically preferred configurations.25 Steps running perpendicular to the Ti rows (parallel to the [11̅0] direction) are not observed,23 as expected from autocompensation considerations. Not surprisingly, the reactivity of UHV-prepared surface is dominated by unsaturated Ti atoms either on the terraces or at O vacancy sites produced by the sputtering and annealing process. In contrast, rutile surfaces immersed in water have a fully saturated structure with no “dangling bonds” or undersaturated sites.26,27 In solution at a pH of 5.8, the surface is uncharged, becoming negatively charged at higher pH and positively charged at lower pH.28 This behavior is explained by two competing reactions: the deprotonation of H2O bound to the exposed Ti rows (pKa ∼ 9) and the protonation of the bridging O sites (pKa ∼ 1), where the quoted acidities are based on molecular simulation.29,30 Figure 1b is therefore a better representation of a rutile (110) surface in aqueous solutions. Nothing is known about the preferred structure, if any, of rutile steps in aqueous solutions or about the acidities of the different step and defect sites.

spectroscopies, necessitating a different approach. In this study, we use a combination of morphological probes and atomistic simulations on rutile (110) to reveal the nature of the reactive site and the reactive species. Most surface science investigations have studied the reactivity of “clean” rutile (110) surfaces prepared by sputtering and annealing in UHV. The structure of the UHV-prepared surface, sketched in Figure 1a, is determined primarily by

2. EXPERIMENTAL SECTION Polished 5 mm × 10 mm rutile (110) samples (MTI Corp.) were reduced by a short vacuum anneal (8 min at 660 °C typ.) before use to induce sufficient conductivity for subsequent scanning tunneling microscopy (STM). Fresh or previously used samples were immersed for varying durations in aqueous solutions comprised of varying volumetric ratios of 30% H2O2 (aq., J. T. Baker, CMOS grade), 30% NH4OH (aq., EMD, ACS grade), 50% NaOH (aq., J. T. Baker, electronics grade), 38% HCl (aq., J. T. Baker, ACS grade), and ultrapure water (Millipore Milli-Q) held at 80 °C in a water bath. In the following, we refer to 1:1:2 H2O2/NH4OH/H2O as “basic peroxide solution.” For extended reactions, the H2O2containing solutions were replaced every 10 min because of rapid H2O2 degradation. The reacted surfaces were briefly rinsed in ultrapure water. Prior to STM imaging, adsorbed water on the sample backsides was blotted dry while water on the front sides was allowed to evaporate, then the samples were transferred to a UHV chamber through a load lock. The samples were heated in vacuum (base pressure ∼2 × 10−10 mbar) to 230 °C for 30 min to desorb the copious quantities of water adsorbed to the surface and sample plate. Thermal desorption of H2O from rutile (110) has been studied extensively.31,32 This mild heat treatment does not induce morphological changes in the substrate, as verified experimentally, although heat treatments at significantly higher temperatures (e.g., 400 °C for 30 min) do induce O vacancies and step mobility. The samples were then imaged at room temperature with a tungsten tip in a UHV STM at a typical bias of 1.80 V and 1 nA tunneling current. The samples were also analyzed by infrared spectroscopy in the reflection geometry and by X-ray photoemission spectroscopy (XPS) in the same chamber as the STM analysis. The pH of fresh basic peroxide solutions was 10.7 at 20 °C as measured with a pH meter (Orion 290A), decreasing to 9.6 after heating in a 80 °C water bath for 11 min. At 20 °C, the pH of a 1:3 solution of H2O2/H2O was 6.2. For some experiments, samples with a low density of irregular, typically monolayer-deep pits (vacancy islands) were prepared by thermal etching of new or previously reacted samples in UHV at 660 °C for 8 min.

Figure 1. Molecular models of rutile (110) surface with an arbitrarily shaped pit bounded by bulk-terminated (unreconstructed) steps, with Ti in light blue, O in μ2 and μ3 oxo bridges in red, and O in aquo ligands in blue. The green Ti−O bonds are ∼2% longer than the gray Ti−O bonds in bulk rutile. A translucent (110) plane is included to distinguish the atomic planes and accentuate step structures. (a) Structure of UHV-prepared surface showing rows of undercoordinated Ti atoms parallel to the [001] direction. The preferred steps are parallel to the [001] and ⟨11̅1⟩ directions. The clean ⟨11̅1⟩ step reconstructs (not shown). (b) Structure of surface in aqueous solution (protons not shown because of pH dependent protonation). After reaction in basic peroxide solutions (vide infra), the preferred steps are parallel to the [001] and [11̅0] directions.

autocompensation21,22 (charge neutrality) and consists of rows of singly unsaturated Ti atoms separated by rows of saturated Ti atoms connected by bridging O atoms. When viewed in a STM, the unsaturated Ti sites image as bright rows, leading to a characteristic alternating row morphology. The UHV annealing process produces relatively straight steps preferentially aligned parallel to the [001] and ⟨11̅1⟩ directions.23,24 In other words, the steps are preferentially parallel to the Ti rows or at a ±65.5° angle to the rows. This preferential orientation allows the step 27344



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3. RESULTS 3.1. Production of Near-Ideal Morphologies. When rutile (110) surfaces were reacted in basic peroxide solution at 80 °C for extended durations (∼20 min or longer), atomically flat surfaces were produced as shown in Figure 2 and Figure S1

High-resolution scans of the Ti 2p region, seen in Figure 3, showed that the sample was fully oxidized with no reduced Ti

Figure 2. STM image of rutile (110) surface after reaction in basic peroxide solution at 80 °C for 20 min showing preferential production of steps parallel to the [001] and [11̅0] directions. The height histogram is inset, confirming the formation of atomic-height steps.

Figure 3. High-resolution XPS spectra of rutile (110) surface after reaction in basic peroxide solution showing (a) the Ti 2p region and (b) the O 1s region as a function of annealing temperature. The spectra were normalized to the (a) Ti 2p3/2 and (b) O 1s bulk transition, and a small energy correction (∼0.05 eV) applied to offset mild band bending.40

of the Supporting Information. While the average step direction was determined by the local miscut, the reacted steps consisted primarily of relatively straight segments parallel to the [001] and [11̅0] directions. Pits (vacancy islands) were rarely observed. The formation of crystallographic patterns on the reacted surface is definitive evidence of anisotropic (i.e., sitespecific) etching. This reactivity is not entirely unexpected, as hot basic peroxide solutions have also been used as titanium etchants33 and to selectively dissolve rutile nanocrystals from anatase−rutile mixtures.34 The production of atomically flat terraces separated by straight or nearly straight steps is characteristic of highly anisotropic (site-specific) etching reactions that attack kink sites much more rapidly than step sites and step sites much more rapidly than terrace sites.35 In other words, if the etch rate of site i is denoted by ki, the criterion for the production of relatively straight steps is

defects, such as Ti3+. The dominant features in the spectra were assigned to Ti4+ transitions: Ti 2p3/2 at 459.2 eV and Ti 2p1/2 at 464.9 eV. The spectra show no evidence of transitions at ∼2 eV lower binding energy than Ti4+, which would be characteristic of reduced Ti species,36−39 such as Ti3+. (While dissociative H2O adsorption masks reduced Ti defects at room temperature, the defect band reappears after H2O desorbs at temperatures ≥500 K.38) A fractional monolayer of reduced Ti species, attributed to O vacancies, are often observed on samples cleaned by sputtering and annealing. The absence of O vacancies on aqueous processed surfaces is not surprising, as Ti3+ is readily oxidized by even mild oxidants, such as O2 (g).32 High-resolution scans of the O 1s region, such as those shown in Figure 3b, were dominated by a transition at 530.5 eV which was assigned to O in TiO2.40,38 In aqueous solution, a H2O molecule will bond to each unsaturated Ti atom, giving rise to a transition at ∼3.5 eV higher binding energy. No evidence of these species was observed, which is consistent with previous research showing rapid desorption of these weakly associated molecules at room temperature.38,41 In contrast, a temperature-dependent shoulder at 2.0 eV higher binding energy corresponding to 0.07 ML of adsorbed O atoms (see Supporting Information) was observed. Transitions at this energy on UHV-cleaned surfaces are typically ascribed to OH bound to O vacancies; however, this assignment is inconsistent with the Ti 2p spectra. Instead, the similar concentration (0.05 ML) and desorption temperature of CO fragments measured from the C 1s spectrum suggest the shoulder is primarily due to

k kink ≫ kstep ≫ k terrace The question is what is the structure of the kink site? The step site(s)? And what chemical reaction is occurring? 3.2. Chemical Analysis. XPS analysis confirmed the production of near-ideal TiO2 surfaces with few defects. Only transitions arising from Ti, O, and C atoms were observed in survey spectra. The amount of C on the surface ranged from barely detectable to a fractional monolayer and was attributed to adventitious contamination during sample transfer. No Ncontaining species were observed. See Supporting Information for more detail. 27345



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adventitious C-containing species. The observed binding energy shift is also in good agreement with known metal peroxo species (e.g., peroxotitanium compounds42) and with shifts in binding energy observed between metal oxides and peroxides,43 so we cannot rule out a small density (∼0.01 ML) of surface-bound peroxo ligands. In principle, vibrational spectroscopy could distinguish peroxides from other O-containing species. Surface peroxo species44 have been identified on nanocrystalline TiO2 by an infrared absorption band at 943 cm−1. Our attempts to observe this transition have been stymied by small temperature fluctuations that perturb the population of a bulk phonon mode of similar energy. These fluctuations caused erroneous background subtraction which mimics the previously reported transition, preventing definitive analysis. 3.3. Pit Shapes Reveal Anisotropic Reactivity. To probe the nature of the reactive sites, rutile (110) surfaces with a low density of irregular nm-scale etch pits were prepared by thermal etching, as shown in Figure 4a. This process is known to evolve

Figure 5. STM images of a rutile (110) surface after reaction in basic peroxide solution at 80 °C for 2 min, showing the relative orientation of the rectangular etch pits and the Ti rows. The region shown in (b) is indicated by the rectangular box in (a). The images in Figures 2−4 were obtained on different crystals.

magnification, atomically straight rows with a characteristic 0.65 nm spacing were observed on the reacted terraces as shown in Figure 5b. These features were assigned to Ti rows parallel to the [001] direction in analogy with UHV-prepared surfaces. The long sides of the rectangles were parallel to the [110̅ ] direction, whereas the shorter sides were aligned with the [001] direction. After ∼5 min of reaction, many highly rectangular etch pits were observed (e.g., Figure 4b). The density of etch pits decreased with further reaction. After ∼15 min of reaction, only a very few pits remained (Figure 4c). This morphological evolution is indicative of highly anisotropic, step-flow etching. The characteristic transformation of the etch pits provided insight into the relative reactivity of step sites. Simple geometric arguments45,46 predict that etching reactions will transform convex structures, such as three-dimensional spheres or twodimensional islands, into geometric structures bounded by fast etching directions (i.e., planes or steps). Similarly, concave structures, such as hollows (3D) and pits (2D), will be transformed into structures bounded by slow etching directions. The production of rectangular etch pits indicated that steps parallel to the [001] direction were significantly more reactive than steps parallel to the [11̅0] direction. We have not been able to conclusively identify the structure of the etched steps from STM images. 3.4. Kinetics Identify Active Species. When initially pitted surfaces were immersed in a 80 °C 1:3 mixture of stock H2O2/H2O that was adjusted to pH 10.9 by dropwise addition

Figure 4. STM images showing the temporal evolution of a rutile (110) surface during reaction in basic peroxide solution at 80 °C. The three images were taken on the same sample, albeit in different locations, and are at the same scale. (a) The initial sample with irregular pits produced by thermal etching, (b) after 5 min of reaction, and (c) after 15 min of reaction.

oxygen and produce a Ti rich surface,22 as evidenced by the low density of white protrusions (excess Ti) on the surface. While not evident in the low magnification image of Figure 4a, this process is also known to produce a significant concentration (few percent) of O vacancy sites. When initially pitted surfaces were immersed in basic peroxide solution at 80 °C, characteristic morphological changes were observed, as illustrated by Figures 4 and 5. After ∼2 min of reaction (Figure 5), the pits increased in size and took on a nominally rectangular habit. At high 27346



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to many-orders-of-magnitude increased reactivity of adjacent ligands. The electronic origins of these effects are beginning to be understood with the aid of molecular simulations.54,56 Adjacent Ti atoms were assumed to be bound together through the appropriate μ2- or μ3-oxo bridges, whereas any remaining bonds were to terminal (μ1) ligands of unspecified type. This is equivalent to assuming that the oxidation of Ti3+ (i.e., O vacancies) was very rapid in solution, consistent with our XPS analysis. Our approach was to start with the simplest possible model and add complexity as needed for consistency with experiment. The simplest possible reaction would be one that targets sites with a single μ1 ligand, such as an undersaturated terrace Ti. This cannot explain the observed morphologies, though, as the reaction does not attack flat terraces. Next in increasing complexity would be a reaction that requires two μ1 ligands on a single Ti atom, such as the “kink site” in Figure 6a. On the basis of geometrical considerations

of stock NaOH, the same morphological evolution was observed at approximately the same rate. Thus, NH4+ was not specifically involved in the reaction. When initially pitted surfaces were immersed in 1:3 mixtures of stock H2O2/H2O (pH = 6.2) at 80 °C, similar morphological changes were observed, albeit at a much reduced rate. Measurements of average pit area as a function of time suggest a ∼100-fold rate reduction from the basic peroxide solution (pH ∼ 10.7). The decomposition of H2O2 is rapid and pHdependent, which makes a more accurate determination of the reaction kinetics difficult. When initially pitted surfaces were immersed in 1:3 mixtures of stock NH4OH/H2O or stock HCl/H2O at 80 °C for 30 min or more, no morphological changes were observed. These observations are consistent with a base-catalyzed etching reaction by H2O2. Base-catalyzed formation of early transition metal peroxo compounds is common and has been proposed to proceed through a hydroxo−hydroperoxo complex.14 While these transient complexes have not been directly observed, their existence is supported by molecular simulations.14 These observations are inconsistent with direct (elementary) reaction with the hydroperoxyl anion, HO2−. While the rapid etching of titanium in basic peroxide solutions has previously been explained in terms of direct reaction with HO2−,17,33,47 the pKa of H2O2 (11.62)48 suggests that concentration of this species should drop by more than a factor of 104 as solution pH drops from 10.7 to 6.2. A direct, rate-limiting reaction by HO2− is not consistent with the observed kinetics. 3.5. KMC Simulations Identify Structure of Reactive Site. Kinetic Monte Carlo simulations were used to identify possible reactive sites on the surface. The model assumed that the surface reaction removed individual Ti atoms randomly at a rate determined solely by bonding geometry (vide infra). The model treated these rates as adjustable parameters. During the reaction, the model assumed that Ti atoms were always bound to six ligands in the near-octahedral geometry appropriate to rutile. Since step atoms on UHV-prepared surfaces are immobile indefinitely at temperatures relevant to aqueous reactions, we assumed that the steps did not reconstruct significantly during reaction. The model adopted the solid-onsolid approximation,49,50 which disallows the formation of overhangs or inclusions. This assumption is consistent with the extremely flat observed morphologies. Diffusion of Ti atoms and deposition of Ti from the solution were also forbidden. The reaction of a semi-infinite surface was modeled using screw boundary conditions51 using the same algorithm, albeit a very different geometry, as previous models of silicon reactivity.52,53 The chemistry of metal oxo coordination complexes, small molecule analogues of rutile, provided a useful framework for classifying the many possible reactive sites on rutile surfaces.26,54 The general expectation is that unbridged or “μ1” ligands, such as the H2O ligands in Figure 1b, will have the highest reactivity. For example, terminal H2O or OH− ligands exchange in less than 300 μs in aqueous Ti4+ solutions.55 Next in reactivity are the singly bridged ligands, such as the μ2-oxo site in Figure 1b, which exchange almost 2 orders of magnitude more slowly in aqueous Ti(IV) species. Ligands that are bound to three Ti atoms, such as the μ3-oxo sites, are expected to have the lowest reactivity. Importantly, the reactivity of individual ligands is expected to be dramatically affected by the nature and configuration of neighboring ligands. In particular, there are numerous examples where deprotonation of a H2O ligand leads

Figure 6. Molecular model illustrating (a) possible structures of step and kink sites, and (b) hypothesized bidentate bonding of peroxo ligand (O22−, purple), suggested by X-ray diffraction of small molecule analogues.63,64

alone, this is a natural candidate for the kink site, in part because any island must contain at least a few sites of this type. The implications of this type of reactivity are shown on the sample morphologies in Figure 7. If sites with two (or more) μ1 ligands were the only ones to react, initially rough steps parallel to the [001] and [11̅0] directions would etch to straight steps, whereas circular pits would etch to a square habit. While this 27347



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one μ2 oxo bridge. Two regimes are shown. In the fast kink etching regime where the rate of kink site etching is much faster than step site etching (in Figure 8, kkink = 103 kstep), the etch pits evolve to a square habit with sides parallel to the [001] and [110̅ ] directions, while the steps etch to a relatively straight morphology. In the slow kink etching regime (in Figure 8, kkink = 10 kstep), the etch pits evolve to a rough circular habit, while the steps adopt a rough, meandering morphology. Importantly, both regimes are self-propagating; they do not stop etching, except on a perfectly flat surface. Clearly, the fast kink etching regime captures most of the essential features of rutile (110) etching except one: the rectangular habit of the observed etch pits. The rectangular habit of the etch pits implies that the reaction preferentially attacks steps parallel to the [001] direction. Within this simple model, this suggests that the reaction distinguishes between the two different types of μ2 oxo bridges: the symmetric μ2 oxo bridges which have two equatorial Ti bonds (gray bonds in Figure 1) and the asymmetric μ2 oxo bridges, which have one axial (green in Figure 1) and one equatorial bond. (Note that all μ3 oxo bridges have the same structure.) These two types of bridges are expected to have significantly different geometries. Neglecting possible relaxations, the included angle of a symmetric μ2 oxo bridge is 98.9°, which is significantly smaller than the 130.5° included angle of the asymmetric μ2 oxo bridges. The simulations in Figure 9 show the effects of distinguishing between the two types of μ2 oxo bridges within the fast kink

Figure 7. Kinetic Monte Carlo simulations illustrating “kink” reactivity on three different initial morphologies. In these simulations, the only reactive sites were kink sites: Ti atoms with two μ1 ligands. The parallel white bars indicate the orientation of the Ti rows which run parallel to the [001] direction and are invisible at this magnification. The reaction stops at the final morphologies; the surface does not continue to etch.

simple reactivity pattern captures some features of the observed morphologies, it fails in one crucial aspect: the square etch pits and the perfectly straight steps are themselves completely unreactive. As a result, etch pits grow to squares, then stop etching. Sites with (at least) two μ1 ligands are, thus, candidate kink sites, but other reactive step species must also be present. This suggests that some sites with a single μ1 ligand are chemically reactive, but if so, what distinguishes a reactive step site from an unreactive terrace site other than the μ1 ligand? One possibility, inspired by the reactivity of metal oxo coordination complexes, is that the etchant distinguishes the sites on the basis of the oxo bridges. Terrace Ti atoms with a single μ1 ligand also have five μ3 oxo bridges to the substrate. These sites are unreactive. In contrast, there are a number of possible configurations of step Ti atoms that have a single μ1 ligand and one or more μ2 oxo bridges. Two such possibilities are shown in Figure 6b. The simulations in Figure 8 test the possibility that the reactive step sites are Ti atoms with one μ1 ligand and (at least)

Figure 9. Kinetic Monte Carlo simulations illustrating the effects of varying the reactivity of step sites with symmetric and asymmetric oxo bridges in the fast kink etching regime (kkink = 104 kstep). For fast symmetric step etching, ksym step = 100 kasym step, whereas for fast asymmetric step etching, ksym step = 0.01 kasym step. The parallel white bars indicate the orientation of the Ti rows which run parallel to the [001] direction.

etching regime (kkink = 104 kstep in Figure 8). As expected, if the reaction is faster at step sites containing a symmetric μ2 oxo bridge (here, ksym step = 100 kasym step), rectangular pits with long axes parallel to the [001] direction are produced, similar to the pits observed experimentally. Conversely, if the reaction is faster at step sites containing an asymmetric μ2 oxo bridge (here, kasym step = 100 ksym step), rectangular pits with long axes parallel to the [11̅0] direction are produced.

4. DISCUSSION 4.1. Reaction Site Specificity. A simple aqueous reaction can be used to produce highly homogeneous rutile (110) surfaces characterized by flat terraces and straight atomic-height steps. Importantly, the steps produced by this reaction have a different orientation, a different structure, and a different reactivity than those produced in UHV, which emphasizes the importance of studying surface reactivity in relevant environ-

Figure 8. Kinetic Monte Carlo simulations illustrating the two different regimes of step-kink reactivity: fast kink etching (kkink = 103 kstep) and slow kink etching (kkink = 10 kstep). “Step” sites are Ti atoms with one μ1 ligand and (at least) one μ2 oxo bridge. The parallel white bars indicate the orientation of the Ti rows which run parallel to the [001] direction. Simulations of surfaces with steps parallel to the [001] direction (not shown) show the same effect. 27348



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ments. The perfection of these chemically prepared surfaces makes them an ideal starting point for studies of rutile reactivity in aqueous environments. The major morphological structures observed in the experiment are captured by a simple, site-specific model of rutile (110) etching that is based on metal oxo chemistry and incorporates three essential features. First, the most reactive sites are those with two (or more) μ1 ligands. While we call these sites “kinks,” the model distinguishes these sites solely on the basis of the ligand count, not by their location on the surface. Second, sites that have one μ1 ligand and one (or more) μ2 oxo bridges are also reactive, albeit at a much lower rate than kink sites. We call these sites “step sites,” but again they are distinguished solely by ligand count, not location. Finally, step sites that contain a symmetric μ2 oxo bridge are significantly more reactive than step sites containing an asymmetric μ2 oxo bridge. The high reactivity of sites with symmetric oxo bridges is likely explained by the rutile lattice itself. The symmetric oxo bridges come in pairs, forming a rhombic Ti2O2 motif with a very short O−O distance of 2.53 Å. Computational investigations show that the strong repulsive interactions between the two oxide anions in this pair locally destabilize the lattice.57,58 In the bulk lattice, this repulsion is counterbalanced by other attractive interactions on both sides of the rhombus. When the rhombus is at a step edge, half of these stabilizing interactions are removed. Both the kink and the step sites are classified on the basis of two bonds: either two μ1 bonds (kink site) or one μ1 and (at least) one μ2 bond (step site). One might question whether the relative orientation of these bonds is also important. For example, is a kink site with two μ1 bonds in a cis configuration more or less reactive than a kink with μ1 bonds in a trans configuration? In our simulations, the rutile geometry dictates that the vast majority of the etching sites are in the cis configuration for both steps and kinks. We have found no morphological indicators of relative cis/trans reactivity in this system. These reactions do not occur on flat rutile (110) terraces, as evidenced by the extremely flat terraces produced by long reaction times (e.g., Figure 2). From kinetic Monte Carlo simulations, we estimate that kink sites are at least 6 orders of magnitude more reactive than terrace sites and possibly much more. 4.2. Peroxo Ligands and Reactivity. What chemical reaction is occurring at these sites? The low density of the reactive sites, which we estimate at 1% (steps) and 0.01% (kinks) of a monolayer from the STM images, coupled with the transient nature of etch intermediates precludes direct spectroscopic observation; however, the reactivity of molecular analogues and the site specificity provide insight into the reaction. Role of Hydrolysis and Condensation. The aqueous reactivity of Ti4+ is dominated by extremely rapid condensation of aquo complexes to form polynuclear species with multiple oxo bridges. The hexaquo cation, Ti(H2O)64+, is not observed, being unstable to spontaneous deprotonation. At very low pH, one or more mononuclear species appear to be stable, such as TiO(H2O)52+ or Ti(OH)2(H2O)42+.55,59 At all but the lowest pH values, these compounds rapidly condense to form oxo bridged species. The rate of the condensation reactions are highly pH-dependent, displaying both acid- and base-catalyzed pathways.60

Condensation, as well as other trends in metal oxide chemistry, can be explained semiquantitatively in terms of the relative electronegativities of the metal, O, and H using the partial charge model.60,61 Over the course of the condensation reaction, the calculated “partial charge” on the Ti atom evolves from 0.98 in Ti(H2O)64+ to 0.88 in Ti(OH)2(H2O)42+ to 0.77 in TiO2 (s). This decrease in local charge favors the condensation reaction. In contrast, acidic solutions favor more protonated (less negatively charged) species. The competition between the two is captured by the partial charge model. Importantly, the condensation reaction is reversible and relatively rapid. NMR studies have shown that the oxygen atom in μ2 oxo bridged titanium species exchanges on the 10 ms time scale at room temperature.55 This rapid exchange raises an obvious question. Does etching occur by the chance simultaneous hydrolysis of all oxo bridges at a particular site? By themselves, concentrated NH4OH and HCl solutions had no effect on the morphology, so the reaction must be more than simple hydrolysis: the hydrogen peroxide plays a crucial role. Role of Peroxide and Peroxo Ligands. The addition of H2O2 to Ti4+ (aq) leads to the formation of titanium peroxo complexes.62 At very low pH, one or more mononuclear peroxo species [e.g., Ti(O2)(OH)(H2O)4+] are stable; however, these species slowly condense at higher pH to form polynuclear species bound by multiple μ2 oxo bridges. In these compounds and many others, the peroxo (O22−) ligand binds in a side-on, bidentate geometry with both O atoms being coordinated to the Ti atom, as confirmed by X-ray diffraction.63,64 Peroxo species have also been observed spectroscopically on nanocrystalline TiO2 surfaces.44,65 The formation and high stability of the titanium peroxo bond can be rationalized both in terms of local charge reduction at the cation by the highly charged peroxo ligand (partial charge model) and by the bidentate bonding. Because of this, the titanium−peroxo bond is stable in aqueous solutions, as confirmed by the original experiments62 and studies of small molecule analogues.15 In this experiment, hydrogen peroxide is a source of peroxo ligands, and the added base catalyzes the peroxo addition reaction.14 This affects the reaction in two ways. First, the addition of one or more peroxo ligands at a Ti center will decrease the partial charge at the cation, increasing the lability of the remaining bonds,20 and accelerating hydrolysis and exchange of the other ligands. Second, small molecule studies have shown that H2O2 can directly attack μ2 oxo bridges. Recent studies of dititanium complexes with two symmetric μ2 oxo bridges show that reaction of H2O2 can either replace a μ2 oxo bridge with a peroxo ligand66 or simultaneously cleave both μ2 oxo bridges.67 The addition of hydrogen peroxide to the reaction also removes the need for the simultaneous hydrolysis of all oxo bridges, enabling a sequential etching mechanism. When an oxo bridge is hydrolyzed, substitution of OH− by O22− blocks recondensation. The reverse reaction, removal of the peroxo ligand, is very slow. As a result, peroxo ligands are widely used to slow and control the condensation of molecular precursors in the synthesis of TiO2 nanocatalysts with controlled architecture.14,68 This mechanism explains the high reactivity of sites with two μ1 bonds (kink sites) in a number of ways. First, base-catalyzed condensation occurs preferentially at metal sites with high 27349



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partial charge,60,69 and the kink site has a higher partial charge than other sites on the surface. Second, the kink site has fewer oxo bridges to cleave, and it is also less sterically hindered than other sites. This mechanism also explains the preferential reactivity of sites with one μ1 ligand and one μ2 oxo ligand (steps) over μ1 sites with no μ2 oxo ligands (terraces), as μ2 oxo bridges hydrolyze much more rapidly than μ3 oxo bridges.26 Nevertheless, the μ3 oxo bridges must react at some point. Otherwise, the Ti atoms would never leave the surface. The cleavage of μ3 oxo bridges appears to be accelerated by prior reaction of the μ2 oxo bridges to form peroxo or hydroxo species. This type of ligand-accelerated labilization is common in metal oxide chemistry,26 and hydroxo ligands are known to have a particularly strong effect.54 Within the context of the partial charge model, this effect can also be understood in terms of charge stabilization by the addition of charged, electronegative ligands. 4.3. Implications for TiO2 Growth. This experiment conclusively shows that peroxide increases the lability of other bonds at the Ti center, leading here to etching of bulk rutile. If the only role of peroxide were to block condensation at the peroxo ligand site, no etching would have been observed. This enhanced lability is strongly supported by studies of metal oxo chemistry, where the addition of a peroxo ligand has been shown to stabilize five-coordinate Ti clusters in solution over the more common six-coordinate species.20 This effect is important in etching, where the near-simultaneous cleavage of multiple bulk bonds is required. The implications for TiO2 growth are clear. Enhanced lability adds a degree of reversibility to growth reactions, favoring the formation of well ordered crystals over amorphous materials. Titanium-peroxo ligands are known to play an important role in polymorph-specific nanocrystal growth, photocatalyzed oxidation (e.g., for self-cleaning surfaces) and in small molecule catalysis (e.g., olefin oxidation70). Nevertheless, our understanding of the surface chemistry of these ligands, particularly at defect sites, is still in its infancy. 4.4. Comparison to Other Rutile Studies. In closing, we note that other researchers have also reported morphological changes to rutile (110) surfaces after solution processing. Most of these studies71−76 combined solution processing with a subsequent high-temperature treatment that is known to anneal and smooth the surface. As a result, the final morphologies reflected the convolution of chemical and thermal processes. Nanometer-scale morphological changes of rutile(110) surfaces after 5 min exposure to pure H2O or concentrated base (NH 4 OH) solutions at room temperature have been reported;77 however, we could not reproduce these changes. Roughening of initially smooth rutile (110) surfaces during anodic photoetching in acidic solutions has also been reported.74 The morphology of rutile (110) surfaces also plays an important role in determining photoreactivity.78 Recent studies have shown that the photoluminescence quantum yield of rutile (110) is dependent on the orientation and angle of the surface miscut as well as the surface roughness, suggesting that step morphology plays an important role in carrier lifetime.

subsequent studies of rutile reactivity in technologically relevant environments. This reaction produces steps with a different orientation, different structure, and different reactivity from those prepared in UHV environments. These surface morphologies can only be explained by highly defect-specific surface reactions that specifically target kink sites and differentially attack step sites. The preferential production of surface steps oriented parallel and perpendicular to the Ti rows, the [001] and [110̅ ] directions, can be explained by a simple model based on metal oxo coordination chemistry. The preferential production of rectangular etch pits is explained by the relative reactivities of symmetric and asymmetric μ2 oxo bridges and rationalized in terms of the structural differences between the two sites. In these experiments, the principle role of the peroxo ligand is to destabilize neighboring bonds and increase their lability. On rutile, this destabilization leads to highly site-specific etching. During growth, the peroxo ligands will do more than just slow down condensation; they will add a degree of reversibility to the growth reaction, favoring the formation of well ordered crystals over amorphous materials.

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ASSOCIATED CONTENT

S Supporting Information *

Further description of STM and XPS analysis and kinetic Monte Carlo simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-607-255-3040. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Erik S. Skibinski for measuring the infrared absorption spectra of chemically prepared TiO2 surfaces. This work was supported by the National Science Foundation (NSF) under Award No. CHE-1303998 and made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC Program (Grant DMR-1120296).

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REFERENCES

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5. CONCLUSIONS The base-catalyzed reaction of H2O2 with rutile (110) produces highly homogeneous surfaces characterized by flat terraces and straight atomic-height steps, an ideal starting point for 27350



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