ARTICLE pubs.acs.org/JPCC
Facilitated Lattice Oxygen Depletion in Consolidated TiO2 Nanocrystal Ensembles: A Quantitative Spectroscopic O2 Adsorption Study Michael J. Elser and Oliver Diwald* Institute of Particle Technology, Friedrich-Alexander University Erlangen-Nuremberg, Cauerstrasse 4, Erlangen, 91058 Germany
bS Supporting Information ABSTRACT: On TiO2 nanocrystal powders, pressure-induced consolidation and subsequent annealing generates mesopores inside the pellet and leads to the depletion of specific surface area. This loss corresponds to the introduction of particleparticle interfaces and grain boundaries. In previous work, they were found to be susceptible to facilitated lattice oxygen depletion as compared to free particle surfaces during vacuum annealing. For the first time, we determined the adsorption of molecular oxygen on these consolidated and nonstoichiometric nanocrystal samples. As a function of repeated cycles of vacuum annealing and O2 exposure, the amount of paramagnetic and diamagnetic surface oxygen species was determined using mass spectroscopy in combination with FT-IR, electron paramagnetic resonance (EPR), and UVvisNIR diffuse reflectance.
’ INTRODUCTION In polycrystalline transition metal oxides, 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. Related phenomena and their dependence on the materials’ properties such as size, structure, and purity of the grains are critically important for a broad spectrum of applications that involve solid-state electrolytes, sensing devices, and materials employed for printable electronics.13 For TiO2, a prototype material for defect characterization studies on transition metal oxides, the bulk and surface defect structure depends on the degree of nonstoichiometry,48 and annealing at low oxygen partial pressures induces lattice oxygen depletion according to: ΔT
TiO2 s f TiO2χ
ð1Þ
ΔT 1 O2 s f O2 þ 2e p < 106 mbar 2
ð2Þ
p < 106 mbar
At small departures from stoichiometry, TiO2χ particles can be characterized as a solid solution of point defects that are either oxygen vacancies or Ti3+ or Ti4+ interstitials.4,5,8,9 At larger deviations, the Magneli series of ordered structured compounds with a common formula TinO2n1 derive from stoichiometric TiO2 by crystallographic shear mechanisms.1012 For many processes in heterogeneous catalysis, nonstoichiometric and oxygen-deficient TiO2 has turned out to be beneficial. As an example, TiO2χ supported metal-based catalysts often show advantages over the fully oxidized material on the basis of strong metal support interactions (SMSI).13,14 Relevant for photocatalysis, on the other hand, optical absorptions in the r 2011 American Chemical Society
range of visible light arise from vacuum annealing of the oxide material and in principle could allow for interfacial charge transfer processes induced by excitation at energies below the band gap.15 On this basis, self-doping procedures of TiO2 are expected to provide materials that utilize a larger fraction of the solar spectrum.16 Moreover, electronic conductivity enhancement as a result of oxygen deficiency makes nanocrystalline transition metal oxides also attractive for photoelectrochemical applications.17 At elevated temperatures, oxygen addition to vacuum annealed TiO2χ allows for oxygen incorporation into the lattice upon restoration of stoichiometry. In the absence of oxidizing agents, rapid cooling of the vacuum annealed TiO2χ powder to room temperature provides a situation where ion transport through the lattice is essentially quenched. Subsequent oxygen addition at room temperature and below initiates interfacial electron transfer between the particle and physisorbed O2 upon formation of anionic chemisorbed species such as O2, O22, or O. These species and their reactivity impact on the overall performance of catalysts as well as solid oxide fuel cells, where the mechanistic steps for the incoming O2 molecule to adsorb, diffuse, dissociate, and ionize before O2 ion incorporation into the lattice need to be understood in detail.1820 The electronic properties of nanocrystalline metal oxide ceramics can differ substantially from those of conventional microcrystalline materials as a result of reduced specific grain boundary impedance. Strongly enhanced oxygen nonstoichiometry and electronic conductivity have been observed over a wide range of oxygen partial pressures.21,22 Moreover, it was Received: September 9, 2011 Revised: December 14, 2011 Published: December 14, 2011 2896
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The Journal of Physical Chemistry C found that ionic conductivity decreases with decreasing grain size, while electronic conductivity increases. These trends have been ascribed to reduced defect formation energies as well as to the presence of microstrain at solidsolid interfaces.22,23 Despite a large amount of electrochemical evidence for annealinginduced stoichiometry changes, there are, to the best of our knowledge, no molecular spectroscopy studies that address a quantitative description of point defects participating in the overall mechanism. Moreover, for nanostructured metal oxide systems, the degree of annealing-induced nonstoichiometry and electronic reduction is usually undetermined. This is reasoned by the complex interplay of many difficult-to-replicate material properties, which include size, particle morphology, adsorbate coverages, and, often neglected, the extent of particle aggregation. As a matter of fact, when discussing why the spectroscopic properties and surface reactivity of nanocrystal ensembles differ from those of macroscopic materials, their high specific surface area, the size and shape of the nanoparticles, and the high concentration of coordinatively unsaturated surface elements are commonly taken into account. The influence of particle particle interfaces,2426 the concentration of which is significantly enhanced in nanocrystalline systems, is, however, strongly neglected in the discussion of their properties. Using a combination of UVvisNIR, FT-IR, electron paramagnetic resonance (EPR), and mass spectroscopy, the present study focuses on the quantitative description of oxygen adsorption on nonstoichiometric TiO2 nanocrystal interfaces present in compressed and vacuum annealed nanocrystal powders. Apart from the determination of the total number of adsorbed oxygen in diamagnetic and paramagnetic form, we also measured the fraction of paramagnetic species to establish a connection between oxygen adsorption and spectroscopic oxygen related fingerprints observed on vacuum annealed TiO2 powders. This study provides first combined FT-IR and EPR spectroscopic evidence for facilitated lattice oxygen depletion at solidsolid interfaces present in consolidated TiO2 nanocrystal ensembles.
’ EXPERIMENTAL SECTION TiO2 nanocrystals were prepared by the metal organic chemical vapor synthesis technique (MOCVS) based on the decomposition of titanium(IV) isopropoxide at T = 1073 K in a flow reactor system. The details of this technique are given elsewhere.27 For purification, the obtained powder samples were subjected to thermal treatment under high vacuum conditions. First, the powder sample was heated to T = 873 K using a rate of r e 5 K min1. Subsequent oxidation with O2 at this temperature followed by cooling in O2 atmosphere was successfully applied to remove organic remnants originating from the precursor material, on the one hand, and to guarantee the stoichiometric composition of the oxide, on the other. UV-diffuse reflectance measurements were carried out at room temperature using quartz glass cells that guarantee vacuum conditions better than p < 106 mbar. The UV-diffuse reflectance spectra were acquired using an integrating sphere in combination with a Perkin-Elmer Lambda 900 UVvisNIR spectrophotometer and then converted to absorption spectra via the KubelkaMunk transform procedure. The nitrogen sorption isotherms were obtained at T = 77 K using an adsorption porosimeter (Micromeritics ASAP 2020). Prior to analysis, samples were outgassed for 6 h in the degas unit of the adsorption apparatus at 473 K under vacuum.
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Figure 1. (a) Instrumental setup for the determination of oxygen uptake, and (b) sample holder for FT-IR and uptake experiments adapted from ref 28.
The BET surface area was evaluated using adsorption data in the relative pressure range p/p0 = 0.050.2. For EPR measurements, the powder sample was contained within a Suprasil quartz glass tube connected to an appropriate high vacuum pumping system (p < 106 mbar). X-band EPR measurements were performed on a Bruker EMX 10/12 spectrometer using a Bruker ER 4102ST standard rectangular resonant cavity in the TE102 mode. The g values were determined on the basis of a DPPH standard. For quantitative measurements, the spin concentrations were obtained by double integration of EPR signals, which were measured at T = 77 K using a microwave power of 200 μW. At this value, both types of paramagnetic species Ti3+ and O2 ions are not saturated, and differences in signal intensities due to differences in the relaxation time can be ruled out. The shape of the O2 ions’ signals can be reliably approximated with spectrum simulation and allows for a more precise double integration of the resulting curves. On the basis of an earlier quantitative O2 photoadsorption study,27 (Supporting Information) the obtained integral intensity values were translated into concentrations of superoxide anions. For transmission FT-IR-spectroscopy, a high vacuum cell developed by J. T. Yates Jr. and co-workers28,29 was used and for this purpose aligned in the optical path of the IR beam of a Bruker Tensor 27 spectrometer system (Figure 1a). The resolution was 3 cm1, and 200 interferogram scans were averaged to guarantee a 2897
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The Journal of Physical Chemistry C reasonable signal-to-noise ratio. Using a hydraulic press, TiO2 nanocrystal powder pellets were produced by uniaxial compression of typically 3.6 mg of sample powder (∼1015 particles) with 100 MPa into a tungsten grid, which subsequently was mounted in a high vacuum cell (Figure 1b). This apparatus allows for controlled sample annealing prior to measurement at pressures in the range 108 mbar e p e 103 mbar. The activation procedure of the consolidated powder samples that were subjected to quantitative FT-IR/mass spectroscopic measurements was identical to that applied to compressed powders, which were measured with electron paramagnetic resonance. Vacuum annealing (p < 106) to T = 923 K was performed at a rate of 5 K/min. At this temperature, a dwell time of 20 min was kept prior to subsequent cooling to T = 373 K performed with a rate of 40 K/min. The coupling of the FT-IR setup with a quadrupole mass spectrometer system (QMS) was attained by attachment of a differentially pumped VG SXP 400 to the sample compartment (Figure 1a). For quantitative oxygen uptake measurements, the pressure in the gas line was adjusted to 2.5 102 mbar (∼4 108 mol of oxygen). This amount of gas was afterward expanded into FT-IR sample compartment and sampled with the QMS where the mass signal at 32 amu was recorded as a function of time. The estimate of the overall error provided for the different data points in Figures 4 and 6 takes into account the following sources of error: (i) scattering of spectroscopic data (EPR and FTIR, Figures 4 and 6) by using the standard deviation for measurements of cycles 710, (ii) standard deviation of the linear regression related to the pressure/mass spectrometer signal calibration functions (Figure 5a), and (iii) confidence interval related to the estimated number of particles. This part of the uncertainty depends on the accuracy of powder weight determination and the assumptions made for the particle size.
’ RESULTS AND DISCUSSION
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Figure 2. UVvisNIR spectroscopic fingerprints of consolidated TiO2 nanocrystals (a, b, and d) after vacuum annealing (p(O2) < 106 mbar) at T g 673 K. Addition of oxygen leads to the entire annihilation of all absorption features observed (c and e).
temperature, that is, T = 673 K, contains a corresponding band with significantly less intensity in addition to the monotonic, structureless broad absorption background (gray shaded area) that stems from conduction band electrons scattered at lattice phonons. The intensities of these so-called Drude-absorptions grow with increasing wavelengths and are also proportional to the concentration of conduction band electrons.3538 Apart from a fraction of electrons in the conduction band, another one that also originates from lattice oxygen removal occupies empty d-orbitals of adjacent Ti-cations and, according to eq 3, forms paramagnetic Ti3+ states.39,40 1e
UVVisNIR Absorption Properties. While a powder of
TiO2 nanocrystal agglomerates that are obtained from chemical vapor synthesis and that exhibit a low concentration of solidsolid interfaces essentially retains their stoichiometry during vacuum annealing (p(O2) < 106 mbar) at T = 873 K,26 we have shown in previous work that mesoporous particle networks with a significant concentration of particleparticle interfaces are subject to substantial stoichiometry changes due to annealing.30 The same applies for compressed and pelletized TiO2 nanocrystal powders, the structural properties of which will be described in the next section. For TiO2, it is well documented that annealing-induced deviations from the 1:2 stoichiometry ratio give rise to a variety of interesting optical and electronic effects.4,3133 These include optical absorptions due to (i) free charge carriers in the MIR, (ii) excitations of electrons in shallow trap states (MIRNIR), (iii) dd transitions of Ti3+ ions, which are located either in regular or in interstitial lattice sites (vis) and, last but not least, (iv) the absorption edge due to interband transitions (UV). The absorption spectrum of an ensemble of aggregated and vacuum annealed TiO2 anatase nanocrystals (Figure 2a and d) in comparison to the oxidized samples (Figure 2c and e) contains all of these features. Starting with the MIR region, the maximum at hν = 0.25 eV in the transmission spectrum of vacuum annealed TiO2χ (Figure 2a) is attributed to electronic transitions from shallow trap states into the conduction band.34 A related spectrum acquired on the same sample, which was attained by application of a lower annealing
Ti4þ f Ti3þ
paramagnetic state
ð3Þ
These can result from particular interstitial titanium positions inside the TiO2 lattice that provide a natural polaronic distortion and thus favor the electron localization in the 3d shell.4143 Ti3+ ions are also associated with spin allowed dd transitions in the range of visible light (Figure 2d) at photon energies hν e 3.2 eV. Consequently, they give rise to the characteristic blue color that TiO2 nanocrystals adopt after vacuum annealing.31,32 At the present stage, it is impossible to determine the level of nonstoichiometry from the optical absorption data, because it is difficult to assess the effectively sampled powder volume and to quantitatively account for scattering effects. In the context of the present study, however, it is important to note that upon addition of 10 mbar O2 to vacuum annealed TiO2 nanocrystals at room temperature, the color is drained from the visible and near IR region. The corresponding spectra (Figure 2c and e) are identical to those measured on the oxidized TiO2 samples before vacuum annealing (not shown). In conclusion, all optical manifestations of nonstoichiometry observed on vacuum annealed and, consequently, oxygen-deficient TiO2‑χ nanocrystals are unstable in the presence of molecular oxygen from the gas phase. Their complete extinction indicates efficient electron transfer from the solid to physisorbed oxygen upon formation of anionic adsorbates, that is, O2 and O22 species.4,5,8 Powder Consolidation Related Effects on Microstructure and Available Surface Area. The structural properties of the 2898
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Table 1. Crystallite Sizes, Specific Surface Area SBET, and Phase Composition of a TiO2 Nanocrystal Powder before and after Cold Pressing (100 MPa) into 7 mm Diameter Pellets and Their Subsequent Vacuum Annealing at T = 923 K for 20 mina
a
average crystallite size/nm
SBET/m2 g1
TiO2 powder
11 ( 1 (fSXRD = 143)
130 ( 10
TiO2 pellet
11 ( 1 (fSXRD = 143)
90 ( 10
blocked surface area
phase composition anatase/rutile ratio
40 ( 10
96:4
96:4
Heating rate = 20 K/min.
Figure 3. Pore size distribution of a powder of TiO2 anatase nanocrystals before and after after uniaxial pressing (100 MPa) and heating under vacuum to 923 K.
pelletized samples discussed above will now be described in more detail. Compression of the nanocrystalline powders was carried out by pressure applications at room temperature using a hydraulic press. Table 1 summarizes the XRD data as well as the results obtained by BET nitrogen adsorption at 77 K related to uncompressed and compressed nanocrystal powders after subsequent annealing to T = 923 K (Figure 3). A comparison clearly demonstrates that TiO2 nanocrystal powder pressing and subsequent annealing neither affects the average TiO2 grain size nor the phase composition. However, surface area available for the adsorption of small molecules is substantially depleted from SBET = 130 m2 g1 to SBET = 90 m2 g1 due to powder compaction. The change in the specific surface area cannot be attributed to sintering processes because the crystallite size determined by XRD remains unchanged. Consequently, the loss of ΔSBET = 40 m2 g1 corresponds to blocked surface area and arises from two major contributions: first, compaction inevitably enhances contact area between TiO2 nanocrystals, that is, the concentration of solid solid interfaces.25,26 Second, inner pores with closed openings must also originate from this procedure. Paramagnetic Species. In addition to the optical absorption properties shown in Figure 2, thermal treatment of compressed TiO2 nanocrystals in a vacuum (p < 106 mbar) and at 923 K generates paramagnetic Ti3+ species (Figure 4a). Their spin Hamiltonian parameters have been reported in previous papers.27,44 O2 addition at room temperature and subsequent evacuation leads to the perfect annihilation of the Ti3+ specific resonance upon the formation of another intense signal (Figure 4b) stemming from O2 ions that, on the basis of their EPR parameters,27 were attributed by Murphy et al. to symmetrical species adsorbed either at five coordinate Ti4+ centers or less regular sites adjacent to the anion vacancy.45 It has to be mentioned at this point that variation of the applied oxygen pressure in the range between 0.1 e p e 100 mbar did not affect the yield of paramagnetic species and, apparently, the ratio of paramagnetic to diamagnetic oxygen species as reaction products.
The broad background signal in the magnetic field range where Ti3+ ions resonate renders the quantification of spins difficult and does not allow for a reliable determination of the absolute number of spins.46 For this reason, we only discuss the two above-mentioned Ti3+ species, which were already reported in previous studies and which are attributed to two different types of surface cations.44 On the other hand, the shape of the O2 ions’ signals can be reliably approximated with spectrum simulation and allows for a more precise double integration of the measured EPR signals. On the basis of an earlier quantitative O2 photoadsorption study27 (see also the Supporting Information), the obtained integral intensity values were translated into concentrations of superoxide anions. Figure 4c plots the relative signal intensities of both types of paramagnetic species as a function of the number of activation steps. After a uniform increase in the abundance of both types of species to annealing steps 46, there is a decrease that levels after step 7. Quantitative Description of O2 Chemisorption. Figure 5a shows a typical O2 uptake curve after exposure of a pellet of approximately 5 mg of compressed TiO2 nanocrystals to molecular oxygen. Upon gas admission, the 16O2 pressure, tracked by the differentially pumped QMS mass spectrometer system, reaches a maximum on the time scale of milliseconds. An instantaneous pressure burst is followed by a monotonous decay of the signal intensity, reaching the baseline after 300 s. The QMS signal difference between the oxidized and stoichiometric nanocrystal ensemble and the same sample with annealing induced nonstoichiometry can be obtained by integration (Figure 5b) and related to an absolute number of O2 molecules chemisorbed by particle ensemble at room temperature (further details in the Supporting Information). In the absence of oxygen chemisorption, the integral mass signal corresponds to the total amount of applied oxygen and allows for the calibration of the mass spectrometer signal. For a nonstoichiometric sample where oxygen uptake occurs, the integral mass signal is diminished by the amount of adsorbed oxygen. The difference of the integral signal intensity allows a quantification of the oxygen adsorbed (Figure 5b). Simultaneously, we acquired FT-IR spectra (Figure 5c) indicating on the vacuum annealed sample either the presence or, after O2 admission, the absence of conduction band electrons (Figure 2). Figure 6 plots the total concentration of chemisorbed O2 molecules in comparison to the fraction of paramagnetic species that were tracked as O2 species with EPR spectroscopy (Figure 4b). We also included the integral intensity of the respective FT-IR in this plot. While the total concentration of adsorbed oxygen continuously increases with the number of activation steps and levels after step 6, the concentration of paramagnetic species shows a relative maximum at this point and levels below approximately 4 O2 radicals per TiO2 particle. Lattice oxygen depletion is determined by the reduction enthalpy of the transition metal oxide.48 For samples comprising 2899
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Figure 4. Electron paramagnetic resonance spectra of vacuum annealed TiO2 anatase nanocrystals (a) before and (b) after subsequent O2 addition at room temperature. The diagram in (c) plots the EPR signal intensities related to these spin centers as a function of activation steps. One activation step refers to (i) vacuum annealing to T = 923 K at p < 106 mbar, and (ii) a dwell time of t = 20 min under these conditions, subsequent cooling to T = 373 K with a rate of 40 K/min. After EPR spectrum acquisition at T = 77 K, the powder sample was contacted with 10 mbar O2. An EPR spectrum acquired after subsequent pumping to p < 106 mbar and cooling to T = 77 K shows the presence of surface adsorbed O2 ions (b). For a better qualitative comparison of the relative signal intensities of O2 and Ti3+ species as a function of progressing cycles, the intensities related to the O2 ions in (c) were multiplied by a factor of 4.
Figure 5. Molecular oxygen uptake on pellets of pressed nonstoichiometric TiO2 nanocrystals at room temperature. In the course of O2 admission, the time-dependent intensity of the 16O2 signal is plotted for a vacuum annealed (nonstoichiometric) and oxidized (stoichiometric) sample (a). The difference in integral intensities (b) corresponds to the number of chemisorbed O2,47 Oxygen adsorption leads to the annihilation of the entire MIR absorption (c) originating from electrons in the conduction band as well as shallow trap states below the conduction band edge.
a substantial amount of grain boundaries, it was found that both enthalpies as well as activation energies for oxygen removal are significantly lower than the respective values for bulk TiO2χ.4750 In the present study, adjoined nanocrystals form and share interfaces as a consequence of powder compression (Table 1). These interfaces are associated with sites with local structures, which are different from those of the free particle surface and the particle volume and which are associated with reduced defect formation energies.53 Isolated TiO2 nanocrystals, on the other hand, essentially lack such interfaces and require treatment at considerably higher annealing temperatures (T > 923 K) to induce nonstoichiometry.44 The trends in the integral IR absorption values and, correspondingly, in the concentration of conduction band electrons and those related to the paramagnetic species (Ti3+/O2 ions) reveal that the region at the interface between the nanocrystals is subject to structural changes with
increasing number of annealing steps (Figure 7). In the course of subsequent annealing and oxygen incorporation cycles, low coordinated surface cations that qualify of O2 stabilization54,55 become depleted in favor of those that adsorb oxygen anions in diamagnetic form. Work function measurements can provide important information on the stoichiometry of the outermost surface layer as well as about the presence and thermal stability of oxygen adsorbates thereon.56 On polycrystalline TiO2 samples, presumably of rutile structure, Nowotny et al. concluded from room-temperature measurements that O2 addition to vacuum annealed samples leads to surface adsorbed superoxide anions O2, which subsequently transform into O radicals that remain irreversibly adsorbed at the surfaces.56 In our study, however, we did not evidence O centers. Recently, Green and Yates57 observed on nanosized TiO2 particles of mixed phase (Evonik Industries, 2900
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O2 f O þ O2 4e
O2 f 2O2
Figure 6. Yield of O2 molecules adsorbed (diamagnetic and paramagnetic) in comparison to the fraction of paramagnetic O2 species as a function of activation steps applied. One activation step refers to (i) vacuum annealing to T = 923 K at p < 106 mbar, and (ii) a dwell time of t = 20 min under these conditions, subsequent cooling to T = 373 K with a rate of 40 K/min. After EPR spectrum acquisition at room temperature, the powder sample was contacted by 10 mbar O2.
Figure 7. Schematic presentation that illustrates the trends shown in Figure 6. Surface elements in the contact region between particles contain high energy sites having a sufficient number of low coordinated surface cations (i.e., high energy sites) that stabilize O2 ions after interfacial transfer of electrons from TiO2χ to molecular O2. Annealing-induced structure changes in the neck region between the particles deplete the number of these adsorption sites upon emergence of low energy sites that favor the formation of diamagnetic oxygen species.
Aeroxide TiO2 P25) an IR active OO stretching mode at 1550 cm1 that was attributed to weakly bound O2 species as precursor to oxygen species, which are active in photooxidation reactions. The absence of such a fingerprint in our experiments that were performed on anatase nanocrystals suggests that the respective absorption band is related to rutile crystal surfaces. TiO2χ: Determination of Nonstoichiometry χ To obtain an estimate for the maximum number of electrons that can be picked up from nonstoichiometric TiO2 particles, the following redox steps need to be considered for O2 chemisorption. 1e
O2 f O 2 2e
O2 f O2 2
paramagnetic state
ð4Þ
diamagnetic state
ð5Þ
paramagnetic states
ð6Þ
paramagnetic and diamagnetic states
ð7Þ
diamagnetic states
ð8Þ
Ultimately, one adsorbed oxygen molecule can acquire up to 4 electrons. The uniform particle properties size and structure of the MOCVS grown TiO2 nanocrystals in conjunction with the quantitative O2 uptake data presented above allow for an estimate of the average degree of nonstoichiometry per particle. While XRD indicates an average particle size of 11 nm, we prefer to calculate with a value of 13 nm, which we previously determined from TEM measurements. A spherical anatase nanocrystal with a diameter of 13 nm contains approximately 7 104 O2 ions with 10% of them located at the particle surface. An average uptake of 18 O2 molecules per particle leads to the generation of 4 paramagnetic oxygen species (corresponding to a transfer of 4 e, eq 3) and up to 14 diamagnetic species (eq 5). Assuming a maximum transfer of 4 electrons per O2 molecule adsorbed upon formation of diamagnetic species (eq 8), we expect approximately 60 electrons per particle, which originate from the annealing induced depletion of 30 O2 ions (eq 1) and become available to oxygen adsorption. Consequently, χ is in the range 1 104 e χ e 5 104. Literature on point defect thermodynamics in polycrystalline TiO2χ with larger grains predicts values for nonstoichiometry χ between 106 and 105 under conditions comparable to those of the present study (i.e., 823 K e T e 973 K and oxygen partial pressures below 106 mbar).58 For vacuum annealed rutile single crystals, where Ti interstitials prevail, nonstoichiometries in the range between TiO1.9996 and TiO1.9999 have been reported.59 Apart from its importance in materials’ characterization, the quantitative assessment of nonstoichiometry of particle systems provides an important base for identifying the influence of process parameters and sintering on the optical, electronic, and chemical properties of metal oxide nanocrystal powders. Such quantitative measurements are also important for the design and synthesis of new materials. Approaches like flame spray pyrolysis of reducible oxides SnO2 and TiO2 were shown to provide nonstoichiometric nanoparticulate materials with promising optical and/or electronic properties.6 The utility of extended ensembles of nanoparticles is subject to their heterogeneities in size, aspect ratio, composition, and last but not least heterogeneities in nature and concentration of defects. These are usually discussed as reasons the properties of any given nanoparticle differ from those of the ensemble. Whereas the small size of nanoparticles, their confined volume, and the associated high surface-to-volume ratios give rise to new desirable properties as compared to bulk materials, the interfaces between the particles provide additional functionality.60
’ CONCLUSIONS On reducible metal oxide nanocrystal powders pressureinduced consolidation and subsequent annealing generate mesopores inside the resulting pellet and lead to the depletion of specific surface area. The associated loss corresponds to the introduction of particleparticle interfaces and grain boundaries that were found to be susceptible to facilitated lattice oxygen depletion during vacuum annealing. Quantitative oxygen uptake measurements in conjunction with spectroscopy (FT-IR, 2901
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The Journal of Physical Chemistry C UVvisNIR, electron paramagnetic resonance, and mass spectroscopy) revealed that the interface region between the particles is subject to structural changes in the neck region with ongoing annealing time. Adsorption sites for O2 stabilization become depleted upon emergence of low energy sites that favor the formation of diamagnetic oxygen species. At the same time, the here developed procedure allowed for a quantitative assessment of the degree of nonstoichiometry of these nanostructures. This method provides a base for future investigations that aim at a systematic evaluation of the role granular interfaces, adsorbate coverage, gas atmosphere, as well as light exposure61,62 on stoichiometry changes of highly dispersed transition metal oxides.
’ ASSOCIATED CONTENT
bS
Supporting Information. Further details about the quantification procedures (EPR, MS) and the estimation of related errors. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Part of this work was financially supported by the Austrian Fonds zur F€orderung der Wissenschaftlichen Forschung (FWF - P19702-N20). We gratefully acknowledge support of the German Science 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. ’ REFERENCES (1) (a) Adler, S. B. Chem. Rev. 2004, 104, 4791. (b) Goodenough, J. B. Annu. Rev. Mater. Res. 2003, 33, 91. (c) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. (2) (a) Maier, J. Adv. Mater. 2009, 21, 2571. (b) Bak, T.; Nowotny, J.; Sucher, N. J.; Wachsman, E. J. Phys. Chem. C 2011, 115, 15711. (c) Gurlo, A.; Riedel, R. Angew. Chem., Int. Ed. 2007, 46, 3826. (d) Mercado, C.; Seeley, Z.; Bandyopadhyay, A.; Bose, S.; McHale, J. L. ACS Appl. Mater. Interfaces 2011, 3, 2281. (3) (a) Batzill, M.; Diebold, U. Prog. Surf. Sci. 2005, 79, 47. (b) W€oll, C. Prog. Surf. Sci. 2007, 82, 55. (c) Noei, H.; Qiu, H. S.; Wang, Y. M.; Muhler, M.; W€oll, C. ChemPhysChem 2010, 11, 3604. (d) Ederth, J.; Heszler, P.; Hultaker, A.; Niklasson, G. A.; Granqvist, C. G. Thin Solid Films 2003, 445, 199. (e) Guenther, G.; Schierning, G.; Theissmann, R.; Kruk, R.; Schmechel, R.; Baehtz, C.; Prodi-Schwab, A. J. Appl. Phys. 2008, 104, 034501. (4) Henderson, M. A. Surf. Sci. Rep. 2011, 66, 185. (5) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (6) Teleki, A.; Pratsinis, S. E. Phys. Chem. Chem. Phys. 2009, 11, 3742. (7) Kowalski, P. M.; Meyer, B.; Marx, D. Phys. Rev. B 2009, 79, 115410. (8) Fujishima, A.; Zhang, X. T.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 515. (9) Finazzi, E.; Di Valentin, C.; Pacchioni, G. J. Phys. Chem. C 2009, 113, 3382. (10) Blanchin, M. G. Key Eng. Mater. 1998, 155156, 359–382. (11) Wallis, D. J.; Browning, N. D.; Nellist, P. D.; Pennycook, S. J.; Majid, I.; Liu, Y.; Vander Sande, J. B. J. Am. Ceram. Soc. 1997, 80, 499.
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