Controlling Phase, Crystallinity, and Morphology of Titania

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Controlling Phase, Crystallinity, and Morphology of Titania Nanoparticles with Peroxotitanium Complex: Experimental and Theoretical Insights Manaswita Nag, Sutapa Ghosh, Rohit Kumar Rana, and Sunkara V. Manorama* Nanomaterials Laboratory, Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad-500 607, India

ABSTRACT This Letter presents a detailed experimental and theoretical study to unravel the unique role of hydrogen peroxide in controlling phase and thereby shape and crystallinity of nanocrystalline titania. Analysis of all reaction parameters revealed that the H2O2/Ti ratio (rp) is the predominant factor to obtain crystalline titania with different phase composition. Evolution of phase and crystallite size of the materials is determined from X-ray diffraction and Raman spectroscopy. Transmission electron microscopy images showed phase-related morphology, that is, truncated anatase and rod-like rutile. The solution Raman and Fourier transform infrared spectroscopy study confirmed different bonding pattern in the reaction medium. Complexation of titanium with -O-O- in bidentate manner with C2v symmetry in the reaction mixture is confirmed by the strong Raman band at ∼630 cm-1. Quantum mechanical calculations are performed using density functional theory with B3LYP/LANL2DZ to provide all possible intermediate structures and to predict the probable mechanism leading to the formation of different phases of titania under different experimental conditions. SECTION Nanoparticles and Nanostructures

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Our strategy is to control the reaction parameters to obtain titania of pure anatase and mixed anatase-rutile phase. The samples have been named as TH, TPH, and TP depending on the H2O2 concentration. To trace the progress of the reaction, the starting mixture is monitored by Raman and FTIR analysis before autoclaving. Solution Raman study showed the presence of titanium peroxo complex with C2v symmetry (Ti-O-O, ∼630 cm-1)13-16 along with peaks for peroxide group (∼880 cm-1) (Figure 1). In addition, the presence of CH3-CH-CH3, O-H, C-O, and -CH3 groups from H2O2, H2O, and IPA present in the system are also identified. Because of the complexation and consumption, peaks at 880 and 825 cm-1 are found to change their intensity compared with the blank. FTIR confirms the formation of complex (e.g., O-H of chelated complex, 2400 cm-1; free -O-O- group, 940-980 cm-1; Ti-O-C, 1000-1200 cm-1; and Ti-O-Ti/Ti-O-O, 700 cm-1)6,17 (Supporting Information (SI), Figure S5). After ascertaining the nature of complex, the PTC is subjected to autoclaving, and the resulting product was isolated for further characterization (XRD, Raman, and TEM). The XRD (Figure 2) (comparison with the standard data, anatase JCPDS card no.: 21-1272, rutile JCPDS card no.: 21-1276) and Raman

itania with desired phase is of great importance because of its wide-ranging potential application in the areas of photocatalysis, solar cells, functional coatings, and so on.1,2 Current research focuses on synthetic routes adopting green methodologies to obtain the required phase. From an environmental viewpoint, water-soluble complexes of titanium have proved to be very promising in titania synthesis, but the role of the complexing ligand and the reaction pathway to control the phase and morphology of the formed TiO2 is yet to be understood.3-7 We have made an attempt to use peroxo titanium complex (PTC) as a precursor to obtain titania by a convenient wet chemical method, predict the final structure and phase of TiO2 depending on the route adopted, and support the theoretical calculation. In our study, the stabilization of highly reactive titanium(IV) isopropoxide (TIP) is attained by complexation with H2O2 in water by formation of an orange-colored water-soluble PTC at pH ∼3 that prevents spontaneous uncontrolled hydrolysis.8-11 PTC is a robust complex in aqueous solution and is used as an analytical reagent for H2O2 through a fast reaction to form peroxotitanate intermediates.12 This precursor allows one to minimize the presence of impurity ions in the solution.8-10 As an improvement over the existing method to obtain the desired TiO2 phase starting with PTC by using additional acid, base, or other reagents,3-7 the present method is a clean process with only concentration of H2O2 as the controlling factor.

r 2010 American Chemical Society

Received Date: August 12, 2010 Accepted Date: September 10, 2010 Published on Web Date: September 15, 2010

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DOI: 10.1021/jz101137m |J. Phys. Chem. Lett. 2010, 1, 2881–2885

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resulted in phase-pure anatase, mixed phase, and predominant anatase phase, respectively. Varying the reaction parameters, a specific condition to obtain TiO2 with major rutile component evolves. To understand the reaction mechanism, molecular modeling (DFT calculations) was carried out.20-24 In general, the initial complexation/chelation of the precursor is used to direct the pathway for the formation of rutile or anatase by facilitating the growth of preferred phase, thereby sterically permitting approach of the octahedron based on the spatial effect of the ligand used.6,25,26 The formation of the different crystalline phases here is associated with the decomposition routes of different PTC structures, leading to seeds of different phases. Hence, the structure of PTC is the key factor for different phase formation.3,6,7 The structures of all probable complexes that can be formed in the presence and absence of H2O2 are optimized. Furthermore, the mechanism has been predicted on the basis of energy values and frequencies of each complex. (Tables S8-S10 of the SI). The initial TIP solution in IPA is expected to form a sixcoordinated titanium-hydroxo complex (structure I)8-10 in the presence of H2O (Scheme 1). Titanium ion is known to form 1:1, 1:2, and 1:3 Ti/H2O2 complexes in the presence of peroxide.13,14 The expected predominant Ti-peroxo complexes considered for calculation under the reaction conditions at various H2O2 concentrations have been optimized and given in Scheme 1. We have classified our results as Case I, Case II, and Case III according to the reaction conditions (depending on hydrogen peroxide content). Anatase TiO2 structure is built by edge and face-shared linking of TiO6 octahedra.27,28 Rutile first forms chains by edge sharing of each TiO6 octahedron along the c axis, and cornersharing among these chains leads to a 3D framework.28 Because the placement of the third octahedron is critical to differentiate the formation of anatase and rutile nuclei,25,26 we have optimized structures involving more than two titanium centers to explain their preferential crystallization (mechanism involving up to five titanium complexes is shown in scheme, Supporting Information). It is known that titanium ion first increases its coordination by using its vacant d orbitals to accept oxygen electron pairs from nucleophilic ligands (such as -OH or H2O).10 Consequently, the titanium ion forms an octahedral structure of Ti(O)m(OH)n(H2O)6-m-n(2mþn-4)-. Therefore, as shown in case I, the structure I can stabilize to structure V, as supported by Zhang et al.8 These steps are represented schematically in case I (structures V and VI in the SI). In case I, monometallic species V and VI are forming initially from structure I and are further interacted with each other and rearrange in a nonplanar pattern to form the energetically more stable bimetallic species. The stereochemistry of the bimetallic structure shows that the approach of the third titanium center via edge sharing always results in a nonplanar arrangement. This reaction path thus leads to anatase. This is the step/stage where the polymorphism may be initiated.27,28 The bimetallic structure transforms sequentially to trinuclear, tetranuclear, and so on. Titanium complexes lead to phase-pure anatase (SI). Case

Figure 1. Raman spectra of reaction solutions of the three different sets of experiments TP, TPH, and TH referred as sample compared with the blank (without TIP).

Figure 2. X-ray diffraction patterns of synthesized titania (a) TH175-3, (b) TPH (7.5)-175-3, and (c) TP175-3 synthesized, where ([) indicates anatase and (o) indicates rutile.

Figure 3. TEM images of the samples (a) TH175-3, (b) TPH (7.5) 175-3, and (c) TP 175-3.

data (SI) establish the formation of titania with different anatase and rutile phase composition (SI).18,19 TEM images of the products exemplify the phase-related morphology of the TiO2 nanoparticles, that is, truncated for anatase and rod-shaped for rutile (Figure 3). All of the reaction parameters are optimized, and the overall study establishes our claim that the H2O2/Ti ratio (rp) is the controlling factor in determining the phase composition of the product. The three major reaction conditions (TH, TPH, and TP)


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Scheme 1. Classification of Case I, Case II, and Case III Based on the Reaction Paths Leading to Different Phases with the Optimized (B3LYP/ LANL2DZ) Structuresa

a

For structural details, see Tables S8-S10 of the SI.

II results in mixture of anatase and rutile initiated from II and III, respectively. Because of the rigid geometry of initial structure(II), this system will grow in a nonplanar edge sharing fashion whenever it interacts with the third titanium center following crystallization to anatase phase in a similar fashion, as described in Case I. In structure III, the planar placement of both equatorial peroxo groups favors rutile growth. Here the Ti centers join with each other, leading to nonlinear growth with vertex sharing through oxo bond (-O-) to form the Rutile phase.25,26,29 The stepwise formation of tri, tetra, and penta metallic structures with positive frequency and high energy of stabilization thus finally leads to the favorable rutile crystallization (SI, Scheme S2). Case III leads to predominant anatase phase because of the similarity of structure IV with structure I of case I and structure II of case II. For structure IV, the nonplanar edge sharing arrangement of the peroxo linkages from all three directions leads to organized growth, resulting in a highly crystalline anatase form, as evident from the XRD and TEM results. The significant increase in crystallinity by use of H2O2 in the synthesis of TiO2 could also be because of the compensation of oxygen vacancies and the activation of the lattice oxygen, as previously described by Khan et al.30 We established the crystallinity by applying the Scherrer equation to the major peaks of the XRD data. These data also give the crystallite size and percentage composition of each of the phases, which is comparable to the information obtained from Raman data.13,14,25,26 The estimation of crystallite size using the Scherrer equation is not very suitable because our particles are not spherical; nevertheless, it is useful in obtaining a fairly good estimate. The TEM images also illustrate that use of H2O2 during synthesis results in well-dispersed, highly crystalline particles (average size 30 nm), and the crystallinity is perceived to increase with the increase in H2O2 amount, time, or temperature of reaction (Figure 3). Our studies also reveal that pressure (autoclave) has a definite role in obtaining the desired phase (SI). Similar results for brookite have recently been reported.7 Therefore, DFT calculations unambiguously explain the formation of anatase predominantly at low and high concentration of H2O2 and rutile (max ∼60%) at medium H2O2 concentration in perfect agreement with the experimental results. In conclusion, this is a successful attempt to demonstrate the formation of titania with controlled properties using hydrogen peroxide, which is known to be an environmentally


benign reagent. The use of hydrogen peroxide in the reaction medium resulted in different percentages of rutile and anatase phase in the synthesized titania. The correlation of phase and shape has been established, and the presence of the titanium-peroxo complex with bidentate triangular geometry has been confirmed. The significance of this study is the correlation of the experimental findings with the theoretical modeling that provides a method to propose a plausible mechanism for the reaction. The work can be adapted to synthesize tailor-made materials for specific applications by appropriate modeling of the reactions.

EXPERIMENTAL METHODS All materials, TIP (Lancaster), 2-propanol or isopropyl alcohol (IPA) (Merck), and H2O2 (30 w/w %) (s.d. fine chemicals), were used as obtained. In a typical reaction, when alcoholic solution of TIP (4.5 mL in 50 mL IPA) is added under cold conditions to 100 mL of distilled water, the transparent solution became turbid white. To this, requisite amount of H2O2 in H2O mixture was added dropwise under cold conditions. The white turbid solution changed to dark golden yellow and then orange transparent or opaque solution depending on the amount of H2O2. This solution was then stirred for 15 min in cold and another 15 min at room temperature and then subjected to autoclave for definite times (0.5-15 h) at temperatures (95 to 250 °C) in a hydrothermal setup. After the reaction was complete, the autoclave was cooled, and the product was washed with distilled water, dried, and ground to fine powder. Note. The samples without H2O2 have been termed as TH (TIP in H2O), those with medium H2O2 concentration as TPH (TIP with H2O2 < 30 wt %), and those with excess of H2O2 as TP (TIP with H2O2>30 wt %). The pH values of three different sets TH, TPH, and TP before HT treatment are 4.0, 3.23, and 3.12, respectively, and these pH conditions prevent hydrolysis, avoiding any preprecipitation of TiO2 prior to autoclaving.7 Hazards. Necessary precautions should be taken while handling hydrogen peroxide. Methodology for Computation. Recent studies on titanium species have shown that the widely used B3LYP method with LANL2DZ pseudopotentials for Ti is quite suitable for geometry and property predictions.20 All titanium complex models considered for calculation are based on their existence at different pH8-10 and H2O2 concentration.13,14 The structures

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are fully optimized at the DFT level using the GAUSSIAN 03 software package.21,22 The optimizations are carried out on all possible isomers of Ti and their fragments by using the hybrid density functional B3LYP method, Becke's three-parameter nonlocal hybrid exchange potential with the nonlocal correlation functional of Lee, Yang, and Parr. The LANL2DZ option places double-ζ functions on the valence shells of the heavier atoms, whereas the inner shells are represented by electron core potentials (ECPs), but for the first-row atoms, the LAN2DZ basis is essentially the D95 basis with no ECPs.23 Vibrational frequencies calculated at the B3LYP/LANL2DZ level have been used for characterization of the stationary points and zero point energy (ZPE) corrections. All geometries are characterized as minima (no imaginary frequency) or as transition state (one imaginary frequency). The transitionstate geometries are then used as input for IRC calculations to verify the connectivity of the reactants and products.24

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SUPPORTING INFORMATION AVAILABLE Percentage of rutile phase from XRD; tables containing the observations changing reaction conditions; Raman analysis; TEM images; mechanism and unit cell structure; overall mechanism; Raman analysis of the reaction solution; stacked plot of FTIR spectra of reaction mixture recorded with time of the reaction; assignment of the bands of IR with the functional group present in the solution; theoretical calculations; total energies (a.u.); dipole moments (debye) of the various complexes calculated in this work for Case I: phase-pure Anatase, Case II: mixed phase (anatase and rutile), and Case III: predominant Anatase; and complete author list of the ref 21. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: manorama@ iict.res.in; [email protected]. Tel: 91-40-27193225. Fax: (þ) 91-40-27160921.

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ACKNOWLEDGMENT We thank Dr. L. Satyanarayana for recording

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the FTIR spectra of the samples. M.N. acknowledges CSIR India for a Senior Research fellowship. (18)

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Controlling Phase, Crystallinity, and Morphology of Titania

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