Article pubs.acs.org/Langmuir
Defects in Oxygen-Depleted Titanate Nanostructures Andrea Vittadini* Istituto di Scienze e Tecnologie Molecolari del CNR, via Marzolo 1, I-35131 Padova, Italy
Michael Schirmer, Marie-Madeleine Walz, Florian Vollnhals, Thomas Lukasczyk, Hans-Peter Steinrück, and Hubertus Marbach* Lehrstuhl für Physikalische Chemie II, University Erlangen-Nuremberg, Egerlandstraße 3, 91058 Erlangen, Germany
Alexander Riss Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
Michael J. Elser, Benedikt Schürer, and Oliver Diwald* Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nuremberg, Cauerstraße 4, 91058 Erlangen, Germany S Supporting Information *
ABSTRACT: The identification of defects and their controlled generation in titanate nanostructures is a key to their successful application in photoelectronic devices. We comprehensively explored the effect of vacuum annealing on morphology and composition of Na2Ti3O7 nanowires and protonated H2Ti3O7 nanoscrolls using a combination of scanning electron microscopy, Auger and Fourier-transform infrared (FT-IR) spectroscopy, as well as ab initio density functional theory (DFT) calculations. The observation that H2Ti3O7 nanoscrolls are more susceptible to electronic reduction and annealinginduced n-type doping than Na2Ti3O7 nanowires is attributed to the position of the conduction band minimum. It is close to the vacuum level and, thus, favors the Fermi level-induced compensation of donor states by cation vacancies. In agreement with theoretical predictions that suggest similar formation energies for oxygen and sodium vacancies, we experimentally observed the annealing induced depletion of sodium from the surface of the nanowires.
■
INTRODUCTION Control over the nature and concentration of defects in particulate nanostructures is a major challenge in physics and chemistry of materials. First, the impact of native defects and impurities on the electronic and optical properties of oxide semiconductors is a complex matter and often the subject of controversies.1,2 Experimentally, it is challenging to synthesize materials with defined concentrations of defects or to identify reliable materials processing protocols that lead to the controlled addition of defects. Nanostructured TiO2 gives rise to a variety of forms, such as sheets, scrolls, and rods, which can interconvert into each other by adjusting the chemical environment as well as temperature and pressure.3−5 As these transformations can originate from the modification of both physical and chemical parameters, respective nanostructures not only differ in size, shape, or structure, they can also exhibit variable compositions.6 Essentially they can range from © 2012 American Chemical Society
stoichiometric TiO 2 to metal-ion or proton-exchanged titanates.3 An interesting example is the reversible interconversion between sodium titanate Na2Ti3O7 nanowires (NWs) and protonated titanate H2Ti3O7 nanoscrolls (NSs) that result from pH changes in aqueous dispersions. Even subtle changes in the synthesis conditions can lead to considerable differences in the structural properties.7,8 A comprehensive examination of the relationship between hydrothermal parameters, product morphology, and phase has been reported only recently.9 Anatase-TiO2, lepidocrocite-like (LL) TiO2, and protonated titanates of the H2Ti3O7 type are closely related from a structural point of view, and have been proposed as possible constituents of the titanate NSs.10−12 In fact, spontaneous Received: February 17, 2012 Revised: April 13, 2012 Published: April 16, 2012 7851
dx.doi.org/10.1021/la301129v | Langmuir 2012, 28, 7851−7858
Langmuir
Article
Figure 1. (a−c) Scanning electron micrographs of Na2Ti3O7 NW agglomerates that were dispersed in water, casted, and dried on a Si(111) sample thereafter. (d) Size distribution of Na2Ti3O7 NW agglomerates in aqueous solution determined by SLS.
electrons associated with the vacancy-generated donor states. Zunger developed rules18 that enable one to predict for a particular material system whether n-type doping is favored. The f irst rule states that n-type doping is feasible for materials for which the conduction band minima (CBM) are far from the vacuum level. This situation applies for materials with large bulk-intrinsic electronic affinities. Thus, a simple electronic structure comparison of a given set of materials can be instructive for the identification of the best candidates to develop n-type conductivity. We, therefore, comprehensively explored on titanate NWs and NSs annealing-induced oxygen vacancy formation in conjunction with other compositional changes using electron microscopy and spectroscopy in combination with ab initio density functional theory (DFT) calculations. As a major result, vacuum annealing-induced oxygen depletion in Na2Ti3O7 NWs was found to occur in combination with sodium vacancy formation. The latter type of defect serves as a native electron acceptor and makes n-type doping of these structures difficult. On the contrary, under the same annealing conditions, H2Ti3O7 NSs transform into substoichiometric and electronrich TiO2−x anatase nanorods.
rearrangement from one form into the other is driven by changes in the local water partial pressure.4 The ongoing controversy about the structural properties of this materials class is related to the fact that the nature of the interface solid−gas versus solid−liquidand the adsorption of water in particular determine structure and morphology.13 Sample transfer into the vacuum chamber of a transmission electron microscope is associated with changes at the interface, and the impact of corresponding procedures requires careful documentation. In a previous paper, we explored the thermal stability of H2Ti3O7 and Na2Ti3O7 nanostructures using transmission electron microscopy (TEM), electron diffraction, and Raman spectroscopy.14 On the basis of systematic annealing experiments at reduced oxygen partial pressures and in the temperature range between 273 K ≤ T ≤ 873 K, we found that the hollow H2Ti3O7 NSs collapse upon formation of complete anatase particles with rod-like morphology and relatively flat surfaces. The Na2Ti3O7 NWs, however, were found to retain their structure and morphology up to 873 K. Apart from structure changes, vacuum annealing of reducible transition metal oxides also gives rise to oxygen depletion, which corresponds to the formation of oxygen vacancies and, thus, leads to nonstoichiometry.6 This paper is motivated by the search for simple engineering protocols to generate functional defects that can enhance electronic conductivity or give rise to particular optical and magnetic properties. Like other wide-gap semiconductors, TiO2 exhibits n-type conductivity that is induced by anion vacancies.15−17 While anion vacancies are ubiquitous defects in semiconductors, not all of them are n-type semiconductors. As a major obstacle to the realization of n-type doping, a Fermi-level induced compensation process can occur upon spontaneous generation of cation vacancies that become active as electron-killer defects. They generate shallow acceptor states that can capture
■
EXPERIMENTAL SECTION
In a typical synthesis procedure, 5 g of commercial anatase powder (Alfa Aesar, 99.9%) was treated with 300 mL 10 M NaOH (Roth, p.a.) at T = 373 K under reflux conditions for 48 h. After synthesis, residual NaOH was removed by washing of the sample with distilled water (Millipore Simplicity 185) several times. The obtained product powder consisted of Na2Ti3O7 NWs. To obtain H2Ti3O7 NSs, one part of the sample was treated at room temperature in 0.1 M HCl (Sigma Aldrich, 37% p.a.) and afterward washed with distilled water. As a result, a mixture of finely dispersed H2Ti3O7 NSs and Na2Ti3O7 NW remnants was obtained. Both types of samples were dried under ambient conditions (air and room temperature) for typical periods of 7 7852
dx.doi.org/10.1021/la301129v | Langmuir 2012, 28, 7851−7858
Langmuir
Article
Figure 2. SEM image of Na2Ti3O7 NWs and H2Ti3O7 NSs (a) before and (b) after annealing to T ≈ 910 K. 70 × 80 nm2. Sample heating in the UHV chamber was realized by resistive heating of the doped Si samples on the sample stage. DFT calculations were performed using Quantum ESPRESSO,22 a suite of programs based on the plane-wave pseudopotential formalism. We adopted the Perdew−Burke−Ernzerhof (PBE)23 gradientcorrected exchange-correlation functional in conjunction with a plane-wave basis set and Vanderbilt ultrasoft pseudopotentials.24 The cutoffs for the smooth part of the wave function and the augmented density were 25 and 200 Ry, respectively. Brillouin zones were sampled with Monkhorst-Pack grids of points.25 Geometries were taken from previous calculations.10,13,26 A key point of the present work concerns the comparison of the band edge energies of several systems, which requires an alignment to a common zero energy reference. While for the two-dimensional systems, such as LL-TiO2 nanosheets, this energy can be straightforwardly taken as the flat value of the averaged electrostatic potential in the vacuum region, more care must be taken when bulk systems, i.e., Na2Ti3O7 and TiO2 anatase are considered. In these cases, we referenced the one-electron energies to the vacuum level by performing separate calculations on slab systems, and by aligning the values of the macroscopic average of the Hartree potential in the inner region of the slab and of the bulk systems. To this end, we found that for TiO2 (anatase), a seven-layer (001)oriented slab is needed, whereas for Na2Ti3O7, a slab containing one sheet of material is adequate. To model defects in lepidocrocite nanosheets, a supercell containing 16 stoichiometric units was used. Models of equivalent size were built to study defects in the other systems. For Na2Ti3O7, estimates from single- and double-layer models were found to agree within 0.05 eV. For these calculations, a Γpoint sampling was found to be adequate.
days. The overall drying procedure corresponds to the small removal of water as a dispersing agent. NW agglomeration occurs and balls of entangled NWs resembling hanks of wool form. Typical electron microscopy images of a sample that was redispersed in water and casted on a silicon substrate are shown in Figure 1a−c. A particle size analyzer based on static light scattering (SLS, Malvern Mastersizer 2000) was used to determine agglomerate size in dispersion and revealed a size distribution (Figure 1d) that is fairly consistent with the agglomerate dimensions as viewed by scanning electron microscopy (SEM). For transmission Fourier-transform infrared (FT-IR) spectroscopy, a high vacuum cell developed by Yates and co-workers19 was used and aligned in the optical path of the IR beam of a Bruker Tensor 27 spectrometer system. The spectral resolution was 3 cm−1, and 200 interferograms were averaged in order to guarantee a reasonable signal-to-noise ratio. Using a hydraulic press, titanate sample pellets were produced to uniaxially compress with 100 MPa dried powder (typically 2 mg) into a tungsten grid which subsequently was mounted in a high-vacuum cell. This apparatus allows for controlled sample annealing prior to measurement at pressures in the range 10−8 mbar ≤ p ≤ 103 mbar. For thermal sample activation, the powder sample was heated to T = 873 K using a rate of r ≤ 5 K·min−1. For experiments conducted with SEM and Auger electron spectroscopy (AES),20 1−3 spatula tips worth of the dried material were placed into 4−10 mL distilled and deionized water and treated in a supersonic bath for approximately 30 min. A few droplets of the suspensions were applied on boron-doped Si(111) substrates (size 10 mm × 1 mm × 0.5 mm, resistivity 0.01−0.02 Ω cm). After 2−3 h of drying under ambient conditions, the Si(111) substrates were transferred into an ultrahigh vacuum (UHV) chamber. All SEM and AES experiments were performed in a two-chamber UHV system (Omicron Nanotechnology, Multiscan Lab) with a base pressure better than 2 × 10−10 mbar. The SEM characterization was performed with a UHV-compatible Leo Gemini electron column with an ultimate electron spot size below 3 nm (according to ASTM E98697). A hemispherical electron energy analyzer enables local AES of structures smaller than 50 nm.21 SEM was performed with 15 kV acceleration voltage and a beam current of 400 pA. The depicted SEM images are reproduced with minor contrast and brightness adjustments to increase the visibility of certain structures. Local AE spectra were acquired with the software EIS/ISEM (Omicron) and processed via Igor Pro (WaveMetrics). During AES, the sample was tilted to approximately 25°, and electron beam voltage and current were set to 15 kV and 3 nA, respectively. The AE spectra were acquired in the nonderivative form, and a modified scan method was applied for data acquisition: Instead of irradiating only one spot, a defined small surface area (“scan window”) is repeatedly scanned. Thereby, the required electron dose is spread over the whole scanned area to lower the local electron dose. The advantage of this scan method is first the decrease of electron-induced effects, e.g., electron-stimulated desorption (ESD) or electron-beam-induced heating (EBIH); furthermore, it allows us to control the position in situ while acquiring the AES data. The scan window during spectrum acquisition in our experiments had a size of
■
RESULTS AND DISCUSSION (1). Microscopy and Spectroscopy on OxygenDepleted Titanate Nanostructures. Na2Ti3O7 NWs and H2Ti3O7 NSs are composed of two-dimensional titanate nanosheets. These particularly interesting and versatile building block elements27,28 either form open-ended NSs with spiral cross sections or flat structures such as Na2Ti3O7 nanoribbons and wires.3 The stability of the structures including that of anatase and their interconversion as a function of pH, temperature, and pressure during treatment in aqueous solution was addressed previously.29,30 For the first time, we explored the thermal stability of titanate nanostructures with SEM and AES. A characteristic sample spot showing both structures NWs and NSsin direct contact with the substrate was investigated with SEM before and after annealing to T ≈ 910 K (≈ 0.8 K/min, 12 min at 910 K). A comparison of the SEM micrographs before and after annealing (Figure 2a,b, respectively) reveals that the morphology of the investigated structures is essentially retained. This is consistent with previous TEM and Raman spectroscopy results on Na2Ti3O7 NWs.14 In the case of the H2Ti3O7 NSs, however, 7853
dx.doi.org/10.1021/la301129v | Langmuir 2012, 28, 7851−7858
Langmuir
Article
Figure 3. FT-IR spectra of H2Ti3O7 NSs (a) and Na2Ti3O7 NWs (b) after annealing in vacuum: absorptions below 4000 cm−1 (red color) originate from isolated and H bonded surface hydroxyls. On the other hand, the broad absorptions between 1000 cm−1 and 4000 cm−1 (blue color) originate from electrons in shallow trap states and in the conduction band. Respective spectral changes were induced by O2 admission at room temperature, which leads to interfacial electron transfer from the oxide nanostructures to molecular oxygen.6.
Figure 4. SEM images of Na2Ti3O7 NWs (a,c) and corresponding AE spectra (b,d) showing the Na content before annealing (red lines), after annealing at T ≈ 830 K for 30 min (green lines) and after annealing at T ≈ 830 K for 60 min (blue lines). During annealing, oxygen was dosed to 1 × 10−7 mbar.
NSs is essentially unaffected by transformation to nanorods,14 the change of the bulk structure escapes SEM detection. Annealing-induced changes in chemical composition and electronic structure were addressed with FT-IR (Figure 3a,b) and AES (Figure 4). H2Ti3O7 NSs and Na2Ti3O7 NWs were annealed at high vacuum (p(O2) < 10−7 mbar) to different temperatures and, after cooling to room temperature, were investigated with transmission FT-IR. Corresponding spectra were recorded on vacuum annealed materials before and after exposure to molecular oxygen (Figure 3a,b).16,17 After sample treatment at 473 K, the room-temperature FT-IR spectra (Figure 3a,b)
we previously observed a collapse of the tubular structure (TEM) upon the formation of TiO2 anatase nanorods (Raman) in the temperature range between 673 and 873 K. This apparent contradiction to the SEM evidence in Figure 2, which suggests that H2Ti3O7 NSs are stable up to 910 K, can be explained by the differences in image generation between SEM and TEM. In SEM, the image contrast primarily results from the surface of the investigated sample and thus provides topographic information. In TEM, where electrons are transmitted through the sample, the structure of the sample interior produces the image contrast. While the shape of the 7854
dx.doi.org/10.1021/la301129v | Langmuir 2012, 28, 7851−7858
Langmuir
Article
4c) are in direct contact with the substrate. Before sample annealing, the Na KLL signals were clearly observable in the AE spectra at kinetic energies of ∼950 and ∼990 eV, in perfect agreement with the literature.37,38 The comparison with spectra acquired after annealing at ∼830 K clearly shows that repeated thermal treatment decreases the sodium peak intensity and leads to the overall decrease of the sodium content of the titanate material. We speculate that the stronger reduction of the sodium in the case of the NWs close to the substrate (position 2) is due to the higher local temperature directly at the substrate as well as the more effective diffusion and thus dilution of the alkali metal on the substrate. In the case of the bundles, we also expect that sodium diffuses from the surrounding NWs into the region of interest, leading to less dilution. AES reveals the annealing-induced depletion of sodium from the Na2Ti3O7 NWs. This is consistent with earlier observations.14 In a previous study, we established a direct correlation between the sodium content of Na2Ti3O7 NWs and the intensity of a photoluminescence emission band at 2.4 eV.39 As demonstrated in ref 14, this photoluminescence band suffers substantial intensity decrease upon vacuum annealing of the Na2Ti3O7 NWs. Thus, we can conclude that the loss of PL emission intensity is associated with a sodium content decrease inside these structures. (2). Rationalization of Observed Trends by Ab Initio Calculations. The microscopic and spectroscopic characterization discussed above indicates that oxygen vacancy formation and, as a consequence, the electronic reduction of the TiO2based solid is significantly facilitated in the case of the protonated H2Ti3O7 NSs. To provide information about band structure of the relevant TiO2/titanate nanostructures and the formation energies of possible defects during annealing in vacuum, ab initio calculations were performed. For this purpose, we computed the band structures of bulk Na2Ti3O7 and a selection of nanosheets that can be used as models of TiO2 NSs, such as LL-TiO2, step-3 H2Ti3O7, and anatase-like H2Ti2O5 (Figure 5; see the work of Casarin et al.13). The use of flat models for nanotubes and NSs is justified from the fact that the electronic structure is scarcely affected by the structure curvature.40 The computed valence band maxima (VBMs) and conduction band minima (CBMs) of all these systems, relative to the vacuum level, are compared in Figure 6. The gap value of Na2Ti3O7 (3.2 eV) is slightly smaller than that computed by Xu et al.,41 who, however, applied a scissor operator of 0.4 eV to correct the generalized gradient approximation (GGA) bandgap underestimation. The VBM of LL-TiO2 is 1.8 eV below that of anatase, which is larger than the value of 0.52 eV measured by Sakai et al.42 However, it is worth noting that our calculations refer to an ideal lepidocrocite structure, whereas the material studied by Sakai et al. is strongly nonstoichiometric, having a Ti1−xO2 (x = 0.09) formula. It is particularly interesting to realize that the CBM of all the TiO2 nanosheet models are comparable to that of bulk anatase. Because the latter is known to be intrinsically ndoped by oxygen vacancies, the first Zunger rule implies that TiO2 nanosheets are likely to be n-type as well. In contrast to that, the CBM of Na2Ti3O7 is very high in energy (Figure 6), from which we conclude that Fermi-level induced compensation is favored in the case of Na2Ti3O7. As cation vacancies give rise to acceptor states and experiments indicate sodium depletion in annealed Na2Ti3O7 samples (Figure 4), a sensible interpretation of the theoretical
revealed the presence of isolated and H-bonded hydroxyl groups (red colored bands), which become entirely eliminated after annealing to 673 K. In the case of the H2Ti3O7 NSs, such treatment produces a broad absorption at wavenumbers ν ≥ 2000 cm−1 (blue area in Figure 3a) originating from the excitation of electrons in shallow acceptor states, which in turn result from annealing-induced oxygen vacancy formation.6 The respective absorption gains intensity after annealing to T = 873 K. A monotonous broad absorption background at wavenumbers ν ≤ 2000 cm−1 is attributed to conduction band electrons scattered at lattice phonons (Drude absorptions).31,32 The absorptions in the near-infrared (NIR) and mid-infrared (MIR) regions become annihilated upon O2 addition at room temperature.6,33 Annealing of Na2Ti3O7 NWs to T = 873 K does not produce Drude absorption and results in only a small absorption at wavenumbers ν ≥ 3000 cm−1 (Figure 3b). For a given annealing temperature, the degree of electronic reduction is reflected by the intensities of the optical absorptions in the NIR−MIR region (Figure 3), which is substantially stronger in the case of H2Ti3O7 NSs as compared to the Na2Ti3O7 NWs. This is in line with a previous electron paramagnetic resonance study where we measured electronically reduced Ti3+ centers as paramagnetic point defects.14 For H2Ti3O7 NSs that were annealed to 673 K, we determined a spin concentration of 400 μmol g−1, while in the case of NWs, the corresponding value was found to be significantly lower, i.e., 7 μmol g−1. To investigate annealing-induced stoichiometry changes, AES measurements were performed at different sample spots (Figure 4). In this analysis, particular attention was paid to the development of the O/Ti and the Na/Ti signal ratios. The investigated Auger signals were the oxygen KLL signal at ∼511 eV kinetic energy, the sodium KLL signal at ∼990 eV, and the titanium LMM signal at ∼381 eV (Figure S1, Supporting Information). The related signal intensities (i.e., the peak areas) were determined via linear background subtraction. Annealing was performed in two steps, each for 30 min at T ≈ 830 K. The oxygen pressure in the chamber was kept at p(O2) = 1 × 10−7 mbar during the heating procedure, making the experimental conditions comparable to those of the FT-IR experiments on unsupported powders (see above). Different to the expectation from FT-IR spectroscopy, a comparison of the spectra before and after thermal annealing (not shown) revealed no significant change of the O/Ti ratio and thus no reduction of the oxygen content for both Na2Ti3O7 NWs and H2Ti3O7 NSs. This discrepancy is first explained by the higher sensitivity of FT-IR to the bulk and the possibility that electrons in shallow trap states as well as conduction band electrons also exist in the inner layers of the structures. A second reason is the intrinsic difficulty of precisely determining the oxygen content of a titanium oxide material by a surface sensitive technique using a focused electron beam, which is always hampered by ESD of oxygen (see, e.g., refs 34 and 35). Local AE spectroscopy on the investigated NWs yielded, in addition to the expected chemical composition, i.e., Ti, O, and Na, traces of carbon, which is common for samples previously exposed to ambient conditions. A representative spectrum acquired at position 1 (cf. Figures 1c and 4a) can be found in the Supporting Information (Figure S1). For the exploration of potential sodium depletion effects, we exclusively investigated Na2Ti3O7 NWs by local AES: The spectra shown in Figure 4b,d were taken at two different NW sample spots: While position 1 is located on the top of an agglomerate “ball” (see Figure 4a), which is spatially separated from the heated silicon surface, the NWs at position 2 (Figure 7855
dx.doi.org/10.1021/la301129v | Langmuir 2012, 28, 7851−7858
Langmuir
Article
slab without defects, and Etot(X) is the total energy of the X element in its reference state. In this way, we find that the formation energy of an oxygen vacancy (making reference to the O2 molecule) is in the range between 5.0 and 5.5 eV for all the examined compounds. On the other hand, making reference to the Na bcc phase, we find that the formation energy of a sodium vacancy in Na2Ti3O7 corresponds to 4.3− 4.4 eV and is even lower. This result resonates well with the experimentally observed depletion of sodium and corroborates the hypothesis that a Fermi-level induced compensation process may become active during vacuum annealing of Na2Ti3O7 NWs. In oxide semiconductors, minute changes in the lattice oxygen concentration can significantly affect their chemical and catalytic activity and determine their crossover from ionic to electronic conductivity regimes.45−47 For these reasons, the observed self-compensation effect has important implications for the successful implementation of titanate nanostructures into functional devices. The identification of effective n-type doping routes and the dependence of their success on the materials’ properties such as structure and composition of the underlying nanostructure play a determining role for a broad spectrum of applications that involve solidstate electrolytes, sensing devices, and materials employed for printable electronics.
■
Figure 5. Ball-and-stick sketches of the systems investigated with the DFT calculations, i.e., (a) TiO2 (anatase); (b) Na2Ti3O7; (c) LL-TiO2; (d) H2Ti2O5; (e) step-3 H2Ti3O7. The last three are two-dimensional systems. Ti atoms are blue, O atoms are red, H atoms are white, Na atoms are black.
SUMMARY The following major conclusions about annealing-induced changes in the morphology and electronic structure of Na2Ti3O7 NWs and H2Ti3O7 NSs were drawn: (1) The morphology of Na2Ti3O7 NWs and H2Ti3O7 NSs remains stable up to annealing temperatures of ∼910 K. (2) In comparison to Na2Ti3O7 NWs, H2Ti3O7 NSs are subject to facilitated lattice oxygen depletion and n-type doping, as revealed by FT-IR experiments. (3) Annealing-induced depletion of sodium from Na2Ti3O7 NWs is observed after annealing at T ≈ 830 K (p(O2) = 1 × 10−7 mbar) by AES (Figure 4). This finding is in agreement with the recent observation of intensity reduction of a sodium-specific PL emission process under comparable experimental conditions.14,39 (4) Rationalization of the experimental observations by ab initio DFT calculations reveals for Na2Ti3O7 NWs that in addition to vacuum annealing-induced lattice oxygen depletion, sodium vacancies emerge from vacuum annealing. By capturing electrons associated with the oxygen vacancy-generated donor states, they effectively annihilate shallow donor states.
Figure 6. Calculated alignment of the band edges for TiO2 (anatase), LL-TiO2, step-3 H2Ti3O7, H2Ti2O5 (anatase-like, 2 × 2 adsorption pattern), and Na2Ti3O7. While TiO2 (anatase) and Na2Ti3O7 are bulk phases, the other systems are nanosheets. All band gaps are underestimated as a result of the well-known limitations of the DFT-GGA approach.
■
and experimental data discussed above suggests that oxygen vacancy formation in Na2Ti3O7 is possibly compensated by sodium vacancies. A detailed ab initio study of the defect chemistry of all the models considered in this study is rather complex and requires the calculation of several defects in different charge states and in different environments.43,44 This is beyond the scope of the present work, and we limit ourselves to compare vacancy formation energies Ef(X), simply computed as
ASSOCIATED CONTENT
S Supporting Information *
A representative Auger electron spectrum acquired on Na2Ti3O7 nanowires is given in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org/.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected];
[email protected].
f
E (X ) = Etot(slab − X ) − Etot(slab) + Etot(X )
where Etot(slab − X) is the total energy of the supercell with a vacancy of the X element, Etot(slab) is the total energy of the
Notes
The authors declare no competing financial interest. 7856
dx.doi.org/10.1021/la301129v | Langmuir 2012, 28, 7851−7858
Langmuir
■
Article
(18) Kent, P. R.C.; Zunger, A. Failure of Nitrogen Cluster States to Emerge into the Bandgap of GaAsN with Application of Pressure. Appl. Phys. Lett. 2003, 82, 559−561. (19) Yates, J. T. Jr.; Experimental Innovations in Surface Science; Springer-Verlag: Berlin, 1997. (20) Lukasczyk, T.; Schirmer, M.; Steinrück, H.-P.; Marbach, H. Electron-Beam-Induced Deposition in Ultrahigh Vacuum: Lithographic Fabrication of Clean Iron Nanostructures. Small 2008, 4, 841−846. (21) Schirmer, M.; Walz, M.-M.; Vollnhals, F.; Lukasczyk, T.; Sandmann, A.; Chen, C.; Steinrück, H. P.; Marbach, H. Fabrication of Layered Nanostructures by Successive Electron Beam Induced Deposition with Two Precursors: Protective Capping of Metallic Iron Structures. Nanotechnology 2011, 22, 085301. (22) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; Fabris, S.; Fratesi, G.; de Gironcoli, S.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (23) Perdew, J.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (24) Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892−7895. (25) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (26) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B 2001, 63, 155409. (27) Zhang, S.; Peng, L. M.; Chen, Q.; Du, G. H.; Dowson, G.; Zhou, W. Z. Formation Mechanism of H2Ti3O7 Nanotubes. Phys. Rev. Lett. 2003, 91, 256103. (28) Wen, P.; Itoh, H.; Tang, W.; Feng, Q. Single Nanocrystals of Anatase-Type TiO2. Langmuir 2007, 23, 11782−11790. (29) Nian, J.-N.; Teng, H. Hydrothermal Synthesis of SingleCrystalline Anatase TiO2 Nanorods with Nanotubes as the Precursor. J. Phys. Chem. B 2006, 110, 4193−4198. (30) Tsai, C.-C.; Teng, H. Structural Features of Nanotubes Synthesized from NaOH Treatment on TiO2 with Different PostTreatments. Chem. Mater. 2006, 18, 367−373. (31) Pankove, J. I.; Optical Processes in Semiconductors; Dover Publications: New York, 1975. (32) Szczepankiewicz, S. H.; Moss, J. A.; Hoffmann, M. R. Slow Surface Charge Trapping Kinetics on Irradiated TiO2. J. Phys. Chem. B 2002, 106, 2922−2927. (33) Kolenko, Y. V.; Kovnir, K. A.; Gavrilov, A. I.; Garshev, A. V.; Frantti, J.; Lebedev, O. I.; Churagulov, B. R.; Van Tendeloo, G.; Yoshimura, M. Hydrothermal Synthesis and Characterization of Nanorods of Various Titanates and Titanium Dioxide. J. Phys. Chem. B 2006, 110, 4030−4038. (34) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (35) Walz, M.-M.; Schirmer, M.; Vollnhals, F.; Lukasczyk, T.; Steinrück, H. P.; Marbach, H. Electrons as “Invisible Ink”: Fabrication of Nanostructures by Local Electron Beam Induced Activation of SiOx. Angew. Chem., Int. Ed. 2010, 49, 4669−4673. (36) Walz, M.-M.; Vollnhals, F.; Schirmer, M.; Steinrück, H. P.; Marbach, H. Generation of Clean Iron Nanocrystals on an Ultra-thin SiOx Film on Si(001). Phys. Chem. Chem. Phys. 2011, 13, 17333. (37) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbook of Auger Electron Spectroscopy., 2nd ed.; Physical Electronics Industries, Perkin-Elmer Corporation: Eden Prairie, MN, 1976. (38) Briggs, D.; Seah, M. P. Practical Surface Analysis: Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; John Wiley & Sons, Ltd: Chichester, U.K., 1990; Vol. 1.
ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the German Research Foundation (DFG), which, within the framework of its “Excellence Initiative”, supports the Cluster of Excellence “Engineering of Advanced Materials” at the University of Erlangen-Nuremberg. Computational resources and assistance were provided by the “Laboratorio Interdipartimentale di Chimica Computazionale” (LICC) at the Dipartimento di Scienze Chimiche of the University of Padova. A.R. gratefully acknowledges support by the Austrian Science Fund (FWF): J3026-N16.
■
REFERENCES
(1) Klingshirn, C. F.; Meyer, B.K.; Waag, A.; Hoffmann, A.; Geurts, J. Zinc Oxide: From Fundamental Properties towards Novel Applications; Springer Series in Materials Science; Springer: New York, 2010; Volume 120, DOI: 10.1007/978-3-642-10577-7 (2) Van de Walle, C. G.; Janotti, A. LDA + U and hybrid functional Calculations for Defects in ZnO, SnO2, and TiO2. Phys. Status Solidi B 2011, 248, 799−804. (3) Bavykin, D. V.; Walsh, F. C. Titanate and Titania Nanotubes: Synthesis, Properties and Applications; RSC Nanoscience & Technology Series, No. 12; O'Brien, P., Kroto, H., Craighead, H., Eds.; RSC Publishing: Cambridge, U.K., 2010; ISBN 978-1-84755-910-4. (4) Vittadini, A.; Sedona, F.; Agnoli, S.; Artiglia, L.; Casarin, M.; Rizzi, G. A.; Sambi, M.; Granozzi, G. Stability of TiO2 Polymorphs: Exploring the Extreme Frontier of the Nanoscale. ChemPhysChem 2010, 11, 15550−1557. (5) D’Arienzo, M.; Carbajo, J.; Bahamonde, A.; Crippa, M.; Polizzi, S.; Scotti, R.; Wahba, L.; Morazzoni, F. Photogenerated Defects in Shape-Controlled TiO2. J. Am. Chem. Soc. 2011, 133, 17652−17661. (6) Elser, M. J.; Diwald, O. Facilitated Lattice Oxygen Depletion in Consolidated TiO2 Nanocrystal Ensembles: A Quantitative Spectroscopic O2 Adsorption Study. J. Phys. Chem. C 2012, 116, 2896−2903. (7) Schürer, B.; Elser, M. J.; Sternig, A.; Peukert, W.; Diwald, O. Delamination and Dissolution of Titanate Nanowires: A Combined Structure and in Situ Second Harmonic Generation Study. J. Phys. Chem. C 2011, 115, 12381−12387. (8) Zhu, H.; Y. Lan, Y.; Gao, X. P.; Ringer, S. P.; Zheng, Z. F.; Song, D. Y.; Zhao, J. C. Phase Transition between Nanostructures of Titanate and Titanium Dioxides via Simple Wet-Chemical Reactions. J. Am. Chem. Soc. 2005, 127, 6730−6736. (9) Bavykin, D. V.; Friedrich, J. M.; Lapkin, A. A.; Walsh, F. C. Stability of Aqueous Suspensions of Titanate Nanotubes. Chem. Mater. 2006, 18, 1124−1129. (10) Vittadini, A.; Casarin, M. Ab Initio Modeling of TiO 2 Nanosheets. Theor. Chem. Acc. 2008, 120, 551−556. (11) Riss, A.; Berger, T.; Stankic, S.; Bernardi, J.; Knözinger, E.; Diwald, O. Charge Separation in Layered Titanate Nanostructures: Effect of Ion Exchange Induced Morphology Transformation. Angew. Chem., Int. Ed. 2008, 47, 1496−1499. (12) Mori, M.; Kumagai, Y.; Matsunaga, K.; Tanaka, I. FirstPrinciples Investigation of Atomic Structures and Stability of ProtonExchanged Layered Sodium Titanate. Phys. Rev. B 2009, 79, 144117. (13) Casarin, M.; Vittadini, A.; Selloni, A. First Principles Study of Hydrated/Hydroxylated TiO2. ACS Nano 2009, 3, 317−324. (14) Riss, A.; Elser, M. J.; Bernardi, J.; Diwald, O. Stability and Photoelectronic Properties of Layered Titanate Nanostructures. J. Am. Chem. Soc. 2009, 131, 6198−6206. (15) Bak, T.; Nowotny, J.; Sucher, N. J.; Wachsman, E. Effect of Crystal Imperfections on Reactivity and Photoreactivity of TiO2 (Rutile) with Oxygen, Water, and Bacteria. J. Phys. Chem. C 2011, 115, 15711−15738. (16) Henderson, M. A. A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci Rep. 2011, 66, 185−297. (17) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515. 7857
dx.doi.org/10.1021/la301129v | Langmuir 2012, 28, 7851−7858
Langmuir
Article
(39) Riss, A.; Berger, T.; Grothe, H.; Bernardi, J.; Diwald, O.; Knözinger, E. Chemical Control of Photoexcited States in Titanate Nanostructures. Nano Lett. 2007, 7, 433−438. (40) Szieberth, D.; Ferrari, A. M.; D’Arco, P.; Orlando, R. Ab Initio Modeling of Trititanate Nanotubes. Nanoscale 2011, 3, 1113−1119. (41) Xu, X. G.; Ding, X.; Chen, Q.; Peng, L. -M. Modification of Electronic, Optical, and Magnetic Properties of Titanate Nanotubes by Metal Intercalation. Phys. Rev. B 2007, 75, 035423. (42) Sakai, N.; Ebina, Y.; Takada, K.; Sasaki, T. Electronic Band Structure of Titania Semiconductor Nanosheets Revealed by Electrochemical and Photoelectrochemical Studies. J. Am. Chem. Soc. 2004, 126, 5851−5858. (43) Van de Walle, C. G.; Neugebauer, J. First-Principles Calculations for Defects and Impurities: Applications to III-Nitrides. J. Appl. Phys. 2004, 95, 3851−3879. (44) Zhang, Z.; Wu, P.; Lu, L.; Shu, C. Study on Vacancy Formation in Ferroelectric PbTiO3 from Ab Initio. Appl. Phys. Lett. 2006, 88, 142902. (45) Maier, J. Thermodynamics of Nanosystems with a Special View to Charge Carrier. Adv. Mater. 2009, 21, 2571−2585. (46) Merkle, R.; Maier, J. How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid-State Reaction with SrTiO3 as a Model Material. Angew. Chem., Int. Ed. 2008, 47, 3874− 3894. (47) Tuller, H. L.; Litzelman, S. J.; Jung, W. Micro-Ionics: Next Generation Power Sources. Phys. Chem. Chem. Phys. 2009, 11, 3023− 3034.
7858
dx.doi.org/10.1021/la301129v | Langmuir 2012, 28, 7851−7858
