Determining the Morphology and Photocatalytic Activity of Two

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Determining the Morphology and Photocatalytic Activity of TwoDimensional Anatase Nanoplatelets Using Reagent Stoichiometry Robert Menzel,† Andre Duerrbeck,† Emanuela Liberti,‡ Hin Chun Yau,† David McComb,§,‡ and Milo S. P. Shaffer*,† †

Department of Chemistry and ‡Department of Materials, Imperial College London, London SW7 2AZ, U.K. Department of Materials Science and Engineering, The Ohio State University, Columbus 43210, United States

§

S Supporting Information *

ABSTRACT: Two-dimensional TiO2 anatase nanoplatelets can be synthesized via solvothermal reaction of titanium(IV) isopropoxide in the presence of structure-directing hydrogen fluoride. High resolution electron transmission microscopy (HRTEM), selected area electron diffraction (SAED), and Xray powder diffraction (XRD) show that the resulting nanoplatelets are heavily−truncated, octahedral TiO2 anatase nanocrystals with a large fraction of high-energy (001) crystal facets. Systematic studies provide insight into the underlying reaction pathways and the competing, morphology-determining roles of hydrogen fluoride and water during nanocrystal formation. TiF4 can be used as an additional or alternative fluoride source in hydrolytic systems, allowing the study of markedly higher fluoride concentrations than previously reported, and/or avoiding the use of HF as a starting material. The findings can be plotted on a HF:H2O:Ti ternary diagram to provide guidelines for the control of average dimensions, aspect ratio, degree of truncation and, thereby, fraction of (001) crystal facets. Depending on the composition of the reaction system, oriented attachment of the anatase nanoplatelets along either (001) or (101) facets can be observed. The photocatalytic activity of nananocrystals with different aspect ratios, determined in dye degradation experiments, demonstrates higher activity of the (001) than (101) anatase facets. KEYWORDS: TiO2,anatase, nanoplatelets, nanocrystal morphology, crystal facets

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INTRODUCTION Nanostructured titanium dioxide (TiO2) has been utilized in a broad range of applications, including photocatalytic water splitting,1 photodegradation of organic pollutants,2 photovoltaics,3 gas sensing4 and photochromic devices.5 At the nanoscale, the thermodynamically most stable and catalytically most active polymorph of TiO2 is anatase.6 For synthesis of TiO2 anatase nanocrystals, wet-chemical approaches have proven particularly attractive; hydrolytic and nonhydrolytic reactions of organic titanium compounds enable controlled reactions at moderate temperatures, yielding nanostructures of high crystallinity.7−10 Further, wet-chemical reactions can be easily modified through additives to control particle morphology, size, and surface chemistry. Conventionally synthesized, TiO2 nanocrystals are approximately equiaxed and exhibit a slightly truncated octahedral crystal habit, mainly terminated by the low-surface energy (101) crystal facets of anatase according to the Wulff construction.11,12 Anisotropic TiO2 nanostructures are available through addition of structure directing agents (SDAs), most commonly carboxylic acids and amines, during the synthesis.13 The most common, nonequiaxed morphologies are one-dimensional © 2013 American Chemical Society

TiO2 nanostructures, such as nanorods and nanowires, which have been shown to exhibit improved photocatalytic activity and enhanced photocurrent efficiencies.14 In contrast, twodimensional (2D) morphologies of TiO2 have been considerably less widely studied. TiO2 anatase nanoparticles with a platelet-like morphology and an estimated 47% of (001) facets were originally prepared by the hydrolytic reaction of titanium tetrafluoride, TiF4, as the precursor and aqueous hydrofluoric acid (10 wt %) as a SDA.15 Theoretical calculations confirmed that fluoride can act as a SDA in the synthesis of TiO2 nanocrystals by markedly reducing the surface energy of the (001) anatase facets to below that of the (101) facets.15 However, the relatively large size (lateral dimensions of around 1 μm, thickness about 0.25 μm), low specific surface area and low yield of these original platelet particles severely limit their properties in applications. Subsequently, the hydrolysis of organic titanium precursors, such as titanium tetrabutoxide or tetraisopropoxide, in the presence of 50 wt% aqueous hydrofluoric acid solution was demonstrated to yield anatase Received: March 9, 2013 Published: April 17, 2013 2137

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nanoplatelets (average length around 40 nm, thickness about 6 nm).16 Nonhydrolytic reactions have also been explored in this context. For instance, the reaction of TiF4 in tert-butanol yielded heterogeneous TiO2 materials, with platelet, hexagon, and rhombus-like nanocrystals, depending on the reaction conditions.17 A considerable improvement in product homogeneity and shape control was recently demonstrated by Gordon et al. who report the nonhydrolytic, high-temperature synthesis of anatase nanoparticles from TiF4 and TiCl4 under inert conditions based on the in situ release of HF.18 Most interest in nanoplatelets arises from their unusual crystallography. While equiaxed and uniaxial anatase nanocrystals are mainly terminated by low-energy (101) crystal facets, TiO2 nanoplatelets exhibit a high fraction of (001) anatase crystal surfaces. The predominance of these high-energy crystal surfaces is expected to give rise to unique advantages. For example in dye-sensitized solar cells, the binding of the dye and charge transport across the interface is thought to depend on the exposed TiO2 crystal surface, rendering anatase nanoplatelets interesting candidates for electron transport materials in photovoltaic devices.19 Further, initial studies indicated superior properties in Li-ion batteries, probably because of wider channels on the (001) surface and the small width of the nanocrystals, easing lithium insertion and extraction.20,21 However, most interest so far has been focused on the catalytic properties of anatase nanoplatelets. Theoretical studies have predicted increased catalytic activities of the (001) anatase surfaces because of a higher density of undercoordinated titanium atoms.11,22 While some experimental studies have supported these theoretical predictions,23,24 a recent study reports poorer catalytic activity of nanoplatelets, compared to (101)-terminated TiO2 nanoparticles.18 These conflicting literature findings demonstrate that further, more comprehensive synthetic studies are needed to understand and control TiO2 anatase nanoplatelet morphology. Detailed synthetic morphology control is required to enable systematic studies of nanocrystal properties as a function of particle dimensions and fraction of exposed crystal facets. The reaction of titanium alkoxides in an aqueous solution of the fluoride source is the simplest strategy to obtain 2D anatase nanocrystals. The reaction system studied in this paper consists of only three components, the titanium precursor, water, and the fluoride source, which facilitates investigations of the underlying reaction pathways of this important particle synthesis. The fluoride concentration has been predicted to be the crucial parameter to control nanocrystal anisotropy and the fraction of high-energy surfaces. Although, the reactant stoichiometries utilized across various reports on anatase nanoplatelets vary somewhat, this important issue has not been investigated experimentally in a comprehensive fashion. In fact, only two very narrow compositional ranges have been studied (for an overview of the current literature see Supporting Information, Figure S12). In this paper, the use of TiF4 as additional or alternative fluoride source allows a significantly larger compositional space to be explored, resulting in a systematic map of the stoichiometry dependence of the anatase nanoplatelet morphology. This approach identifies the limitations of nanoplatelet synthesis in the presence of fluoride, and provides a means to control and predict the habit of 2D anatase nanocrystals.

Article

EXPERIMENTAL SECTION

Titanium(IV) isopropoxide (Ti(OiPr)4, TTIP, 97%, Aldrich); titanium tetrachloride (TiCl4, 99%, Aldrich); titanium tetrafluoride (TiF4, Aldrich); aqueous hydrogen fluoride solution (HF, 50 wt%, Aldrich); aqueous hydrochloride acid solution (HCl, 37 wt%, VWR); 2propanol (C3H8O, HPLC grade, VWR), Brilliant Blue R (BBR, Aldrich), and water (H2O, HPLC grade, VWR) were used as-received without any further purification. In a typical experiment (Reaction 1, see Supporting Information, Table S1), TTIP (16 mmol, 5 mL) was placed inside a PTFE liner cup and aqueous hydrofluoric acid solution (50 wt%, 0.6 mL) was added dropwise. After sealing the stainless steel autoclave, the mixture was allowed to react for 24 h at 180 °C inside an electric oven, and left to cool overnight. The resulting white precipitate was washed by bath sonication in 30 mL of 2-propanol for 20 min and, subsequently, centrifuged at 17000 g for 30 min. This washing procedure was repeated three times. After drying in air at room temperature, the typical product was obtained as a white powder in around 80% yield of TiO2 (unless otherwise stated). All reactions using aqueous HF were variations of this general procedure; for the detailed reaction conditions see Supporting Information, Table S1. As an alternative source of structure directing fluoride ions, several experiments were carried out using TiF4 instead of aqueous HF. In a typical experiment (Reaction 14, see Supporting Information, Table S2), TTIP (12 mmol, 3.64 mL), TiF4 (4 mmol, 0.5 g), 2-propanol (16 mmol, 1.22 mL), and H2O (16 mmol, 0.288 mL) were placed inside a PTFE liner cup and reacted in an autoclave for 24 h at 180 °C. The product workup followed the procedure described above. Typically, the product was obtained as a white powder in 70% yield of TiO2 (unless otherwise stated). All reactions carried out using TiF4 as SDA were variations of this general procedure; detailed reaction conditions are listed in the Supporting Information, Table S2. A number of techniques, including transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and thermogravimetric analysis (TGA), were used to characterize the products of the various reactions. For the TEM experiments, samples were dispersed in water, and deposited onto a holey carbon film. TEM morphology and crystallography studies were carried on a JEOL 2010 microscope operating at 200 kV. Mean diameters, lengths, and aspect ratios were obtained from TEM image analysis of around 100 individual structures per sample. High resolution TEM images and SAED patterns were collected on an FEI Titan 80-300 microscope operating at 300 kV. SEM imaging and EDX measurements were carried out on a GEMINI LEO 1525 FEGSEM at an accelerating voltage of 5 kV; dried TiO2 powder was fixed to a SEM stub by a carbon adhesive disc. X-ray diffraction (XRD) measurements were carried out on a Phillips PW1710 diffractometer using Cu Kα radiation with λ = 1.5406 Å. Crystal domain sizes, D, were calculated from the full width at half-maximum, β, of the corresponding XRD peak at 2θ using the Scherrer equation: D=

0.9λ β cos(θ)

TGA was carried out using a Perkin-Elmer Pyris 1 TGA. A (2 ± 0.1) mg portion of TiO2 powder were heated in air (10 mL/min) at 10 K/min between 50 and 850 °C. For further experimental details, refer to the Supporting Information. Photocatalytic measurements were conducted on anatase nanocrystals in their as-synthesized state and after treatment with NaOH. To remove surface ligands present after particle synthesis, the TiO2 powder (100 mg) was bath sonicated in 0.1 M NaOH (20 mL) for half an hour, left to rest for 2 h, and then washed repeatedly with water until neutral. The photodegradation of Brilliant Blue R (0.5 mmol/L; Aldrich) in aqueous solution (pH 10, NH4OH) was carried out in a quartz reactor in the presence of TiO2 nanocrystals (0.2 mg/mL) under the irradiation of UV light (100 W Hg lamp, 365 nm). To overcome particle aggregation (necessary to ensure full accessibility of the 2138

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anatase (001) particle surfaces), the photodegradation experiments were carried out at pH 10 where stabilization of individual anatase nanocrystals is favored through electrostatic repulsion. Prior to irradiation, the dispersions were left stirring in the dark to reach adsorption equilibrium.

microscopy (Figure 1a,b) clearly showed the formation of thin nanoplatelets with lateral dimensions of around 50 nm and thicknesses of about 5 nm, that is, with an average aspect ratio of 10, comparable to previous findings.16 In the HRTEM images of individual nanostructures (Figure 1c,d), the characteristic crystal lattice spacings of 0.19, and 0.24 nm are resolved, corresponding to the (200) and (004) planes of TiO2 anatase, respectively. The different crystallographic planes observed for platelets in face-on and edge-on orientations indicate that the thickness of the nanocrystals is oriented along the z-axis, that is, along the [001] direction of the anatase unit cell. The virtual absence of (004) diffraction spots in the SAED pattern of face-on oriented platelets (inset Figure 1a) and the complementary strong intensity observed in the SAED of edgeon oriented nanostructures (inset Figure 1b) also indicate that the platelet thickness is orientated along the [001] direction for the majority of the particles. Differences in the breadths of the (004) and (200) XRD diffractions peaks (Figure 1g) confirmed the anisotropy and crystallographic orientation on the bulk scale. Scherrer analysis was used to estimate crystal domain sizes. The [001] crystal domain size of 6 nm (calculated from the (004) XRD peak broadening, see Supporting Information for further details) agrees well with the platelet thickness, observed by TEM. However, Scherrer analysis of the (200) peak broadening indicates a crystal domain size of only 25 nm in the plane of the platelets, that is, only about half the platelet length observed by electron microscopy. This discrepancy may be due to the slight bending of the thin platelets (as observed for some of the nanostructures in the TEM edge-on view, Figure 1b), leading to reduced XRD coherence lengths in the [100] direction (for further details see Supporting Information). The (200) broadening may also be affected by lattice strain, a wellknown effect in anisotropic particles. Initial high-resolution, through-focal TEM and diffraction experiments indeed indicate a degree of surface strain in the anatase lattice; however, a more detailed discussion of the issue is beyond the scope of this paper and will be presented elsewhere. Nevertheless, XRD, HRTEM, and SAED characterization give a consistent picture of the crystallographic orientation of the nanocrystals, indicating that the majority of platelets are heavily truncated octahedra with (001) and (101) anatase crystal facets exposed at the platelet faces and platelet edges, respectively. TEM indicates an average base angle of 67° (Figure 1d), consistent with the angle between the (001) and the (101) lattice planes of the anatase unit cell.15 For comparison with previous literature reports, the fraction of the two crystal surfaces can be calculated based on the geometry and the average dimensions of the platelets. Accordingly, the high-energy (001) crystal facets are estimated to make up 82% of the total platelet surface while the fraction of conventional (101) anatase crystal facets is only 18%, comparable to previous findings.12,21 It should be noted, however, that not all of the platelet (001) faces may be available in practical applications as a marked tendency for stacking (Figure 1b) and oriented attachment (Figure 6a) along these high energy surfaces is observed. Further, a minority of the nanostructures exhibit rhombic rather than octahedral geometry (see Supporting Information for further details). Therefore, although frequently calculated in the literature, the (001) crystal surface fraction is only a rough estimate. In fact, the specific surface area determined experimentally by liquid nitrogen adsorption (77 m2/g) is about 35% smaller than the

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RESULTS AND DISCUSSION Synthesis of TiO2 Nanoplatelets Using HF as SDA. As a control, TTIP was reacted with concentrated aqueous hydrofluoric acid at an molar ratio of TTIP:HF:H2O of 1:1:1 (Reaction 1), using similar reaction parameters to those previously reported for the synthesis of TiO2 nanoplatelets. XRD measurements (Figure 1e) confirmed that the white product obtained was highly crystalline TiO2 anatase. Electron

Figure 1. TiO2 anatase nanoplatelets synthesized at standard conditions using HF as SDA (Reaction 1): (a), (b) low magnification TEM bright field images of nanoplatelets mainly orientated face-on and edge-on, respectively; insets show SAED patterns of nanoplatelets in mainly face-on and edge-on orientation (see Supporting Information, Figure S1 for the corresponding TEM images); (c), (d) HRTEM images of single nanoplatelets in face-on and edge-on orientation, respectively (lines mark the (200) and (004) lattice spacings of the TiO2 anatase crystal); (e) XRD pattern of nanoplatelet powder. 2139

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Figure 2. Impact of reactant stoichiometry on TiO2 nanoparticles produced using HF as SDA: (a) Variation of TTIP molar ratio: average particle dimensions and aspect ratios (as determined by TEM image analysis) for TiO2 nanoplatelets produced using 0.33 to 2 equiv of TTIP (Reactions 1 2, 3, 4 and 5); (b) Variation of fluoride molar ratio: TEM images for TiO2 nanoparticles produced using 0 to 1 equiv of HF (Reactions 6, 7, 8); (c) Variation of water molar ratio: particle length histograms (as determined by TEM image analysis) for nanoparticles produced using 1 to 3 equiv of H2O (Reactions 1, 9, 10, 11).

catalyzed) self-condensation of TTIP, for example, via ether elimination,25,26 or the direct reaction between TTIP and HF (alkyl fluoride elimination).27 The 1H NMR spectrum of the washing liquid clearly indicates the formation of diisopropyl ether, that is, the byproduct of the ether elimination pathway, but no signals related to isopropyl fluoride (the expected byproduct of an alkyl halide elimination) are observed. The latter mechanism cannot be excluded, however, as isopropyl fluoride is highly volatile and therefore difficult to detect through standard analytical methods. Another explanation for the surprisingly high yield of the overall reaction might be the release of additional water through titanium-catalyzed side reactions of the 2-propanol28 formed initially. For instance, 2propanol can undergo thermal dehydration and dehydrogenation at elevated temperatures in the presence of TiO2.28 In fact, 1H NMR (Supporting Information, Figure S3) indicates the presence of the corresponding reaction products, namely, branched polypropylene (i.e., polymerized propylene) and acetone. These findings clearly show that the formation of TiO2 nanoplatelets in solvothermal systems is, thus, based on a mixture of reactions rather than a simple one-step hydrolysis: an initial partial hydrolysis of TTIP is followed by secondary reactions that complete the conversion of TTIP into TiO2. Despite the complexity of the underlying reaction system, the dimensions and aspect ratios of the resulting titania nanoplatelets can be controlled through the reactant stoichiometry. Variation of the Relative Reactant Concentrations. To investigate the influence of the TTIP:HF:H2O molar ratio on the morphology, the quantity of each reactant was systematically varied while keeping the other two reactants constant at 1:1 (Reactions 1−11, Figure 2, Table 1). XRD characterization and Scherrer analysis of the (004) and (200) peak broadening generally confirmed the morphology evolution, observed by electron microscopy (for further information see Supporting Information, Figure S9, Table S4). When the molar amount of TTIP was increased

one estimated through simple geometric calculations, further suggesting that a significant fraction of the surface is inaccessible because of stacking. Thermogravimetric analysis in air (Supporting Information, Figure S2) showed that the nanoplatelet surfaces are covered by around 4 wt % of capping agents (fluoride, hydroxyl and isopropyl surface groups introduced by the various reagents). The EDX spectrum (Supporting Information, Figure S2) of the product powder indicated the presence of fluoride, likely stabilizing the (001) surfaces of the platelets in accordance to the structure directing influence of the fluoride ions, predicted by theoretical calculations.15 The primary pathway for the formation of TiO2 in the current reaction system is the hydrolysis of the titanium alkoxide by the water of the concentrated hydrofluoric acid, followed by alkoxolation and condensation to produce the Ti− O−Ti network. To investigate the reaction byproducts, the wet powder cake formed in the reaction was washed with CDCl3 and the washing liquid analyzed by NMR (for further details see Supporting Information). The 1H NMR spectrum (Supporting Information, Figure S3) confirms the presence of 2-propanol, the typical alcohol formed in the hydrolysis of TTIP. HF,180 ° C

Ti(OiPr)4 + 2H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ TiO2 + 2iPrOH

(1)

However, it is worth noting that the complete hydrolysis of alkoxides requires a 1:2 ratio of alkoxide and water. Therefore, the near quantitative TiO2 yield of our standard reaction, carried out at a considerably lower water content (1:1 ratio of TTIP:H2O) strongly suggests that additional reaction pathways, beyond simple hydrolysis, play an important role. The known formation of partially hydrolyzed long chain hydroxyalkoxides, [TiO(OR)2]n, at low metal to water ratios might favor other, nonhydrolytic pathways.8 Potential alternative routes for the formation of Ti−O−Ti networks are the (acid2140

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heavily truncated octahedra with significantly reduced aspect ratios of around 2. Further decrease in HF concentration (Reactions 6 and 7) led to formation of even smaller, near equiaxed particles. These findings confirm the strong structure directing impact of HF, indicating that high HF concentrations are desirable to obtain large, thin nanoplatelets with high aspect ratios. However, an increase of the molar HF amount beyond the 1:1:1 reactant ratio of the standard reaction is not possible when using 50% aqueous hydrofluoric acid (for alternative approaches to increase the fluoride concentration see below). Finally, when increasing the relative amount of H2O (Reactions 9, 10, and 11, Table 1), at constant TTIP:HF ratio, the average particle aspect ratio reduced to about 3−4 (Table 2, Supporting Information, Figure S5). Particle length distributions (Figure 2c) indicated that, upon increasing the relative water concentration from 1 to 3 equiv of H2O, the fraction of smaller, near equiaxed nanoparticles increases markedly while the fraction of the larger nanoplatelets drops sharply and eventually disappears completely at high water contents. Synthesis of TiO2 Nanoplatelets Using TiF4 as the Source of SDA. The observations discussed above suggest that a high, relative fluoride concentration seems desirable to obtain even larger aspect ratios. However, the use of 50% HF limits the HF:H 2 O molar ratio to 1:1. Therefore, titanium tetrafluoride (TiF4), which can release up to 4 equivalents of fluoride per titanium, in situ, was added as a source of additional SDA to the standard reaction with HF (Reactions 14−16, Table 2). An increase in the relative fluoride concentration by about 25% (Reaction 14) resulted in the formation of slightly larger nanoplatelets compared to the standard Reaction 1 (Figure 3a, Table 2). XRD (Supporting

Table 1. Influence of Reactant Stoichiometry on the Morphology of TiO2 Nanocrystals Produced Using HF as SDA molar ratio Ti:F:H2O 1 2 3 4 5 6 7 8 9 10 11 12 13

1:1:1 2:1:1 0.66:1:1 0.5:1:1 0.33:1:1 1:0:1 1:0.2:1 1:0.5:1 1:1:1.5 1:1:2 1:1:3 1:0.5:3 1:1.5:2

particle length (nm)a 50 ± 24 ± 78 ± 74 ± 71 ± 19 ± 12 ± 25 ± 31 ± 28 ± 24 ± 22 ± 140d

12 3 18 19 18 2 2 5 9 8 5 5

thickness (nm)a

aspect ratiob

fraction (001) (%)c

5±1 11 ± 2 5±1 5±1 6±1 19 ± 2 12 ± 2 12 ± 1 10 ± 2 8±2 8±1 10 ± 2 7d

10 2 16 15 12 1 1 2 3 4 3 2 23d

82 45 88 88 85 19 19 43 56 59 55 45 92d

a As determined from TEM image analysis. bCalculated as mean length divided by thickness. cCalculated based on truncated octahedral particle geometry with a base angle of 67°. dDimensions of a second population of large nanoplatelets.

(Reaction 2, Figure 2a, Supporting Information, Figure S4), laterally smaller, thicker particles were formed, that is, the particle anisotropy reduced significantly. In contrast, on decreasing the TTIP concentration (Reaction 3), laterally larger platelets but similarly thin platelets were formed, that is, the particle aspect ratio increased markedly to 16. However, a further decrease in the titanium alkoxide concentration (Reactions 4 and 5) did not result in further increase in aspect ratio, but to a slight reduction of the lateral platelet dimensions (Table 2, Supporting Information, Figure Table 2. Influence of Reactant Stoichiometry on Anatase Nanoparticle Produced Using TiF4 as SDA molar ratio Ti:F:H2O 14 15 16 17 18e

1.1:1.25:1 1.25:2:1 2:5:1 1:1:1 1:1:1

particle length (nm)a 60 ± 66 ± n.a.d 41 ± 44 ±

18 26 14 12

thickness (nm)a

aspect ratiob

fraction {001} (%)c

4±1 5±1 n.a.d 9±3 5±1

15 13 n.a.d 5 9

88 86 n.a.d 67 80

Figure 3. Nanostructures produced when adding TiF4 to the standard Reaction 1 to obtain high fluoride concentrations: TEM images of nanostructures obtained when using (a) 0.125 equiv of TiF4 (Reaction 14); (b) 0.25 equiv of TiF4 (Reaction 15); (c) 1 equiv of TiF4 (Reaction 16); XRD indicates that the particles in (a) and (b) are TiO2 anatase, while the particles in (c) are TiOF2 nanocrystals.

a

As determined from TEM image analysis. bCalculated as average length divided by average thickness. cBased on truncated octahedral particle geometry with a base angle of 67°. dFormation of TiOF2 instead of TiO2. eExperimental conditions as in Reaction 17 but with addition of 0.1 mL of 37% HCl.

Information, Figure S11, Table S4) confirmed the presence of highly crystalline platelets with slightly reduced average thickness of around 4 nm. Further increase in the fluoride concentration (Reaction 15, Figure 3b) yielded platelets with similar dimensions but more irregular morphology, potentially indicating the onset of TiO2 etching. Similar to observations discussed above, etching is accompanied by the formation of significant amounts (∼40%) of TiOF2, as indicated by XRD (Supporting Information, Figure S11). When the content of TiF4 in the reaction system is increased even further (Reaction 16), the product contains virtually no TiO2, as indicated by XRD (Supporting Information, Figure S10). Instead, large, square TiOF2 particles are observed by TEM (Figure 3c), likely to exhibit cubic geometry, based on the cubic unit cell30 of TiOF2.

S4). The corresponding drop in reaction yields (Supporting Information, Table S1) and the formation of small amounts of TiOF2 (as indicated by XRD, Supporting Information, Figure S9) suggest that the high molar ratio of HF (relative to the titanium content) leads to etching of the TiO2 nanoplatelets and the incorporation of fluoride into the Ti−O−Ti network.29 Next, the effect of altering the relative HF concentration at constant TTIP:H2O ratio (1:1) was investigated. Both electron microscopy (Figure 2b, Table 1) and XRD characterization (Supporting Information, Figure S9) indicated a pronounced change in nanocrystal morphology when the molar ratio of HF was reduced. When using 0.5 equiv of HF (Reaction 8), the nanocrystal shape changed from thin platelets to thicker, less2141

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In the limit, the use of TiF4 provides a potential route to avoid the use of free HF as an initial reagent, by allowing it to form in situ. As a starting point, a reaction system of TTIP, TiF4, and water was tested, using a reactant stoichiometry that maintained the Ti:F:H2O molar ratio (1:1:1) of Reaction 1; the other reaction parameters, such as temperature and reaction duration, were retained (Reaction 17, Supporting Information, Table S2). The nanocrystals produced in this reaction were pure anatase and exhibited anisotropic, platelet-like shapes (around 40 nm length and 9 nm thickness), as confirmed by electron microscopy (Supporting Information, Figure S6) and XRD (Supporting Information, Figure S10). However, the particles were more heterogeneous and exhibited a significantly smaller average aspect ratio of 5, than the equivalent standard reaction using hydrofluoric acid. The heterogeneity may arise through a slow release of fluoride from TiF4, that is, the concentration of the SDA remains comparatively low, especially at the start of the reaction. To facilitate a quicker release, the reaction was repeated with the addition of 0.1 mL of concentrated HCl (Reaction 18) to catalyze the hydrolysis of TiF4. The nanocrystals formed (Supporting Information, Figure S6) were considerably thinner with an average aspect ratio of around 9, and, therefore, comparable to the products of the standard reaction (Reaction 1). Thus, nanoplatelets with high aspect ratios can be easily produced in simple, solvothermal reaction systems (that do not require inert conditions or the exclusion of moisture) via in situ release of HF, avoiding the direct handling of hazardous hydrofluoric acid; neutralizing the HF generated in situ, before workup, is relatively straightforward. Although not investigated in this paper, it is very likely that the aspect ratio and crystal facets of nanocrystals produced using a TiF4-based reaction system could be controlled in the same way as described for the HF-based reactions discussed in this paper. Control of Particle Aspect Ratio through Reactant Stoichiometry. The evolution of nanocrystal morphology with changing reactant stoichiometry can be visualized using a ternary diagram describing the relative composition of titanium, fluoride and water in the hydrolytic reaction system (Figure 4). The orange-shaded, central area in the diagram marks stoichiometries that yield pure anatase nanocrystals with 2D anisotropy. The diagram confirms that high, relative contents of HF (beyond 1:2 ratio of F:Ti, marked by green line in Figure 4) are required to obtain platelet-like morphology whereas high concentrations of water (beyond the 2:1 Ti:H2O ratio for complete hydrolysis, marked by the blue line in Figure 4) effectively counter this structure directing effect. A likely explanation is the competition of surface-terminating reactions, involving HF and water, respectively. At high HF:H2O ratios, a large fraction of the surface is likely to be fluoride-terminated. EDX confirms the presence of 3−4 at % fluoride in the nanoplatelets, roughly consistent with estimates for the complete coverage of the (001) surfaces by fluoride. However, the optimum fluoride concentration for platelet formation is clearly far above the stoichiometry needed for surface modification (i.e., excess fluoride in solution is required to promote 2D anisotropy in anatase), indicating an equilibrium between surface-bound and free fluoride. The main equilibria affected by the variation of the relative HF and H2O concentrations at different stoichimetries are the direct reaction of HF with the titanium alkoxide (eq 2) and the fluorination of the growing nanoparticle surface (eq 3):

Figure 4. Ternary diagram illustrating the dependence of TiO2 nanoparticle morphology on the molar ratio of titanium source, fluoride-source, and water; the orange-shaded area indicates the compositional range in which pure 2D TiO2 nanoparticles are produced. The * symbols mark the various reactions carried out; “A” marks the reaction discussed in Figure 5 (Reaction 13).

≡Ti−Oi Pr + HF ⇌ ≡Ti−F + iPrOH

(2)

≡Ti−OH + HF ⇌ ≡Ti−F + H 2O

(3)

As fluoride has been predicted to preferentially locate on the high surface energy (001) facets of anatase, the formation of Ti−F bonds is likely to terminate growth of the nanocrystals in the z-direction of the anatase unit cell, while allowing further lateral growth in the [100] and [010] directions.15,31,32 Consequently, the relative HF concentration determines the thickness, aspect ratio, and available (001) surface area of the nanoplatelets. At higher H2O concentrations, the equilibrium in eq 3 shifts to the left, resulting in a higher fraction of hydroxylterminated TiO2 surface sites that continue to take part in the Ti−O−Ti network formation, thereby, leading to more isotropic crystal growth. The ternary diagram also demonstrates that the solvothermal synthesis of TiO2 nanoplatelets is limited at very high, relative fluoride concentrations where the formation of TiOF2 and soluble titanium fluoride complexes dominates. At such conditions, the TiO2 nanoplatelets reduce in size or even disappear completely through HF etching. However, Figure 4 allows the selection of optimized reactant stoichiometries that provide high HF molar ratios but avoid etching and the formation of unwanted byproducts. For example, at a Ti:F:H2O ratio of 1:1.5:2 (point A in Figure 4), the higher HF content is offset by a slightly increased water content. While a number of the nanocrystals exhibit the dimensions previously observed for the standard reaction, a significant, second fraction of particles with average side lengths between 100 and 200 nm are observed, that is, with aspect ratios up to 20−40, larger than previously reported in the literature. The mixture of larger and smaller particles in this sample suggests that the increased water content in this reaction system leads to a prolonged initial nucleation phase, and, thus, more heterogeneous crystal growth. Oriented Attachment of Nanoplatelets. As described above, the majority of nanocrystals, produced across the different reaction conditions, exhibit morphologies consistent with the truncated octahedral geometry, expected for single2142

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fluorination of the platelet faces, leaving only the hydroxylterminated edges available for oriented attachment, and providing further experimental evidence for the preferential binding of fluoride onto the (001) facets. While control of shape may be challenging, this oriented attachment may be manipulated in the future to create large platelets or assemblies. Influence of Reaction Temperature. To study the effect of temperature, reactions were carried out at 150 and 200 °C, using three typical reactant stoichiometries. Interestingly, the corresponding reaction yields reduced markedly (below 50%) for both increased and decreased temperatures (see Supporting Information, Table S3), indicating an optimum equilibrium of the various TiO2-yielding reactions at 180 °C. At low temperatures, secondary reactions, for example, the catalytic dehydration of 2-propanol, may be significantly inhibited, explaining why, the reaction at 150 °C and high, relative TTIP concentrations (Reaction 24) has an unusually low TiO2 yield of only 3%. In contrast, temperatures higher than 180 °C may accelerate undesired side reactions, such as the dissolution of TiO2 by hydrofluoric acid and TiOF2 formation (as evidenced by XRD, Supporting Information, Figure S11). In fact, at 200 °C and high molar HF:H2O ratios, large, micrometer-sized TiOF2 particles appear next to thin TiO2 nanoplatelets (see TEM, Supporting Information, Figure S8). Both TEM and XRD characterization (see Supporting Information and Figure 7) showed that, independently of the

Figure 5. Anatase nanocrystals prepared at a molar TTIP:HF:H2O ratio of 1:1.5:2 (Reaction 13): (a) TEM image and (b) corresponding length distribution histogram illustrating the formation of smaller (20−100 nm) and larger nanoplatelets (100−230 nm).

crystalline anatase nanocrystals. This morphology suggests that nanoplatelet formation occurs mainly through classical crystal growth, that is, through monomer addition onto anatase nuclei, preferentially in the a and b directions of anatase, depending on the relative fluoride concentration. However, a minority of the particles exhibit unusual, irregular morphologies (Figure 6) that suggest a significant role of oriented attachment. The more frequent observation of irregular shapes at high H2O and high HF concentrations suggests that oriented attachment is promoted at more extreme stoichiometries (however, classical crystal growth remains the main pathway across all stoichiometries.) The oriented attachment mechanism is wellknown for the assembly of 3D and 1D TiO2 structures33 but has not been well studied for anatase nanoplatelets. TEM imaging (Figure 6) suggests that different types of oriented

Figure 6. TEM images illustrating potential oriented attachment of nanoplatelets: (a) nanoparticles produced at a high water concentration (Reaction 12), suggesting oriented attachment along the (001) facets; (b) nanopaletets produced at a high fluoride concentration (Reaction 13), indicating oriented attachment along the platelet edges, that is, (101) crystal facets.

Figure 7. Effect of the reaction temperature on the aspect ratio of TiO2 anatase nanoplatelets at different TTIP:HF:H2O ratios, as determined by TEM image analysis (for further details see Supporting Information).

attachment occur at high molar ratios of HF and water, respectively. High-resolution, through-focal TEM experiments confirm the continuity of the lattice; however a detailed description of these electron microscopy studies is beyond the scope of this paper and will be published elsewhere. When large relative concentrations of water (i.e., small HF:H2O ratios) were used, many particles appeared to have fused along their exposed (001) faces, leading to the formation of thicker platelets with smaller aspect ratios (Figure 6a). Similar observations have been made before, when nanoplatelets, produced in the presence of HF, were treated with water at elevated temperatures.31 Because of the large fraction of OHterminated surface sites at these conditions, particles condense, preferentially along their high energy (001) facets.31 In contrast, at large HF:H2O ratios, some particles seemed to have been formed through oriented attachment along their edges. This observation indicates a high degree of surface-

reaction temperature, the same general trends in nanoplatelet dimensions were observed upon change of reactant stoichiometry, confirming that moderately high relative HF concentrations favor the formation of nanoplatelets with larger aspect ratios. Further, this set of experiments demonstrated that higher reaction temperatures generally yielded longer and slightly thicker platelets, while lower temperatures resulted in shorter platelets with constant or reduced thicknesses. As a consequence, the average platelet aspect ratio significantly decreased when the reaction was carried out at 150 °C, but increased by 1 to 2 units at 200 °C. Higher temperatures tend to favor nucleation and, hence, growth of smaller particles,34 as well as thermodynamic control, leading to more isotropic products.35 Here, the reverse trend suggests that ripening or oriented attachment mechanisms increasingly compete with 2143

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reporting enhanced photocatalytic activity of anatase nanoplatelets compared to the bench mark material P25.24,37 Opposite trends, observed by other authors,18 are likely to originate from testing particle activities in very different reaction systems (e.g., methanol degradation on platinumdecorated nanocrystals) where other catalytic mechanisms apply. Alternatively, insufficient nanoplatelet dispersion, or retained surface functional groups may have reduced accessibility to the (001) surface.

classical crystal growth at higher temperatures. Similar observations have been made for the formation of isotropic TiO2 particles from TTIP and have been related to increased dissolution of primary particles and decreased viscosity of the reaction medium at higher temperatures.36 Photocatalytic Activity. The availability of anatase nanocrystals with different aspect ratios, produced by simple compositional variation of a fixed reaction system (as opposed to the addition of other, potentially confounding, additives), allows an unambiguous assessment of the photocatalytic activity of the particles as function of (001) surface fraction. For this study, the photocatalytic degradation of Brilliant Blue R was selected because of the stability of this organic dye at a wide range of pH values. Three different anatase particles with aspect ratios of 10, 2, and 1, and estimated (001) surface fractions of 82%, 43%, and 19%, respectively, were investigated (products of Reactions 1, 8, and 7, respectively). The kinetics of dye degradation were semiquantitatively assessed in terms of dye half-life times (Supporting Information, Table S6). When used in their as-synthesized state, all particles were found to be photocatalytically active, but only comparatively slow degradation was observed (Supporting Information, Figure S.14). However, removal of inhibiting surface ligands, such as fluoride and isopropyl, via NaOH treatment (resulting in a more uniform, hydroxylated particle surface), resulted in significantly faster photocatalytic dye degradation (Figure 8, Supporting

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CONCLUSIONS This paper provides a systematic study of the formation of TiO2 anatase nanocrystals with 2D anisotropy via straightforward solvothermal synthesis from titanium alkoxide. The nanocrystal anisotropy is induced through structure-directing hydrogen fluoride in the reaction system which can be either added directly or released in situ from TiF4 under acidic conditions, thereby avoiding the direct use of hazardous, concentrated hydrofluoric acid. The simple composition of the reaction system (titanium source, fluoride source, and water) enabled preliminary investigations of the underlying reaction pathways, indicating that the solvothermal synthesis of TiO2 nanoplatelets is based on a delicate equilibrium of various pathways, requiring sufficient activation temperatures to initiate nonhydrolytic reactions. The relative concentrations of fluoride and water are the key stoichiometric parameters to control nanocrystal shape. Moderately high relative concentrations of HF are required to shift the equilibrium of surface-terminating reactions from hydroxylation to fluorination, allowing the structure-directing influence of the fluoride ions to come into full effect. The solvothermal synthesis is limited by competitive side reactions, particularly TiOF 2 formation and TiO 2 dissolution at high HF concentrations or high temperatures. Based on these principles, simple adjustment of reactant stoichiometry and reaction temperature can easily tune the average particle aspect ratio between 1 and 20. Geometric estimates suggest that the fraction of high-energy (001) anatase crystal surfaces can be, thus, varied between 20 and 95%. The use of anatase nanocrystals with variable (001) surface fractions as photocatalysts clearly showed that the (001) anatase facets are considerably more active than (101) crystal facets in the photocatalytic degradadtion of the dye Brilliant Blue R. The detailed synthetic control over the aspect ratio of anatase nanocrystals will enable the study of other fundamental anatase properties and performance in devices as function of the particle surface crystallography. Further, the synthetic guidelines developed allowed the selection of optimized reaction parameters to produce nanoplatelets with lengths up to 230 nm and aspect ratios up to 40, larger than previously reported. The availability of large, thin TiO2 nanoplatelets may be relevant for specific applications, for example, the use of nanoplatelets as filler materials in composites to enhance optical and mechanical properties. We also present first evidence for nanoplatelet growth by oriented attachment. This mechanism might be explored in the future to produce even larger platelet anisotropy or to direct the assembly of the platelets in hierarchical systems.

Figure 8. Photocatalytic degradation of the dye Brilliant Blue R in the presence of NaOH-treated anatase nanocrystals with different fractions of (001) crystal surfaces; the inset shows the decrease of dye half-life time with increasing absolute (001) surface area. Time zero indicates the start of UV illumination.

Information, Table S6). Independent of the surface treatment, the shortest dye half-life time was found for the nanocrystals with the highest aspect ratio and largest fraction of (001) surface (Figure 8). It is noteworthy that the dye half-life times did not correlate with the specific surface areas, SBET, or with the absolute total surface area of the anatase samples (Supporting Information, Figure S15). In contrast, the dye half-life times correlated very well with the absolute, estimated surface area of (001) facets (inset Figure 8), clearly demonstrating that the dye photodegradation is promoted by (001) rather than (101) anatase crystal facets. These findings agree with previous predictions and experimental findings,

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ASSOCIATED CONTENT

S Supporting Information *

Detailed conditions for the various reactions, further experimental details on the characterization methods and data analysis, TGA and EDX characterization of Reaction 1, NMR 2144

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(21) Chen, J. S.; Lou, X. W. Electrochem. Commun. 2009, 11 (12), 2332. (22) Gong, X. Q.; Selloni, A. J. Phys. Chem. B 2005, 109 (42), 19560. (23) Liu, G.; Yang, H. G.; Wang, X.; Cheng, L.; Pan, J.; Lu, G. Q.; Cheng, H.-M. J. Am. Chem. Soc. 2009, 131 (36), 12868. (24) Liu, S.; Yu, J.; Jaroniec, M. J. Am. Chem. Soc. 2010, 132 (34), 11914. (25) Wang, C.; Deng, Z. X.; Zhang, G. H.; Fan, S. S.; Li, Y. D. Powder Technol. 2002, 125 (1), 39. (26) Wang, X. M.; Xiao, P. J. Mater. Res. 2006, 21 (5), 1189. (27) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121 (7), 1613. (28) Bondarchuk, O.; Kim, Y. K.; White, J. M.; Kim, J.; Kay, B. D.; Dohnalek, Z. J. Phys. Chem. C 2007, 111 (29), 11059. (29) Buslaev, Y. A.; Bochkareva, V. A.; Nikolaev, N. S. Russ. Chem. Bull. 1962, 11, 361. (30) Vorres, K.; Donohue, J. Acta Crystallogr. 1955, 8. (31) Yang, X. H.; Li, Z.; Sun, C. H.; Yang, H. G.; Li, C. Z. Chem. Mater. 2011, 23 (15), 3486. (32) Yang, X. H.; Yang, H. G.; Li, C. Z. Chem.Eur. J. 2011, 17 (24), 6615. (33) Menzel, R.; Cottam, B. F.; Ziemian, S.; Shaffer, M. S. P. J. Mater. Chem. 2012, 22 (24), 12172. (34) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46 (25), 4630. (35) Jun, Y. W.; Jung, Y. Y.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 615. (36) Oskam, G.; Nellore, A.; Penn, R. L.; Searson, P. C. J. Phys. Chem. B 2003, 107 (8), 1734. (37) Han, X.; Kuang, Q.; Jin, M.; Xie, M.; Zheng, L. J. Am. Chem. Soc. 2009, 131, 3152.

spectrum of the reaction byproducts of Reaction 1, TEM images, electron diffraction patterns, and XRD spectra for the various samples synthesized, crystal domain sizes for the various samples as calculated from XRD peak broadening. 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: m.shaff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Mustafa Bayazit and Luke X. Reynolds for discussions and help during experiments. The authors would like to thanks EPSRC (grant code EP/G007314/1) for financial support.

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ABBREVIATIONS SDA, structure directing agent; TTIP, titanium(IV) tetraisopropoxide; TEM, transmission electron microscopy; HRTEM, high resolution transmission electron microscopy; SAED, selected area electron diffraction; XRD, X-ray diffraction; TGA, thermogravimetric analysis; SEM, scanning electron microscopy; EDX, energy dispersive X-ray spectroscopy; BET, Brunauer, Emmett, and Teller

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