7605
2006, 110, 7605-7608 Published on Web 03/31/2006
Particles Coming Together: Electron Centers in Adjoined TiO2 Nanocrystals Michael J. Elser,† Thomas Berger,† Doris Brandhuber,† Johannes Bernardi,‡ Oliver Diwald,*,† and Erich Kno1 zinger† Institute of Materials Chemistry, Vienna UniVersity of Technology, Veterina¨rplatz 1/GA, A-1210 Vienna, Austria, and UniVersity SerVice Centre for Transmission Electron Microscopy, Vienna UniVersity of Technology, Wiedner Hauptstrasse 8-10/137, A-1040 Vienna, Austria ReceiVed: February 4, 2006; In Final Form: March 14, 2006
To include particle attachment and porosity of nanostructured materials in the discussion of their electronic properties is critical to our understanding of charge transfer across grain boundaries. We report the condensation of isolated TiO2 nanocrystals via the application of a simple hydration-dehydration cycle. After contact with water and subsequent removal of adsorbed water, these nanocrystals form a mesoporous structure with altered properties as compared with the original material: first, the energy needed for defect formation is substantially reduced, and second, electron paramagnetic resonance measurements reveal the presence of polarizable conduction band electrons not detectable in samples which have not been in contact with water.
A major challenge in chemistry and physics of nanomaterials is to relate the properties of an integral assembly of particles to phenomena which depend on the nanocrystal’s characteristics such as its size, shape, and degree of isolation or attachment.1-3 Due to the high surface-to-volume ratio, the surface crucially affects these properties.4-6 Consequently, changing the interface, for example, switching the surrounding phase from liquid to vacuum conditions, can fundamentally alter the characteristics of the total particle assembly. Nanocrystalline TiO2 is a widely investigated system which is reasoned by many technologically important applications in energy conversion,7-9 photocatalysis,10-12 and sensoric devices.13,14 For photovoltaic applications, nanostructured TiO2 electrodes generate good photocurrents upon illumination. They consist of interconnected nanometer-sized semiconductor particles and comprise significantly modified photoelectrochemical properties compared with compact electrodes. In such devices, the operation relies on a competition of electron diffusion over the whole TiO2 nanoporous film (typically 10 µm thick) and charge carrier recombination in the huge internal surface which is in contact with the solution.7 For this study, anatase TiO2 nanocrystals were produced by the metal organic chemical vapor deposition (MOCVD) method in a flow reactor system.15 (Details about synthesis and sample processing can be found in the Supporting Information.) On such nanocrystals, which adopt an average size of 13 nm, the process of UV induced charge separation was recently investigated by a combination of electron paramagnetic resonance (EPR) and infrared (IR) spectroscopy.16 It was found that at a maximum only one pair of hole and electron can be efficiently separated per crystallite via persistent charge trapping.17 This limit, explained by Coulomb repulsion, together with the * Corresponding author. E-mail:
[email protected]. Fax: 0043-1-250773890. † Institute of Materials Chemistry. ‡ University Service Centre for Transmission Electron Microscopy.
10.1021/jp0607465 CCC: $30.25
observation that charge separation is reversible with respect to temperature18 points to the fact that trapped electrons and holes remain on the isolated particles. Here, we present first combined microstructural and spectroscopic evidence for nanocrystal attachment upon generation of new electron center species. For sample hydration, the nanocrystals were dispersed in high purity grade H2O (Milli-Q) for several minutes. After first drying in air for 18 h, the quartz glass cell was then connected to a high vacuum stage and pumped to P < 10-5 mbar. Sample annealing to 473 K provided TiO2 nanocrystals free of physisorbed water, as judged by IR spectroscopy. As a consequence of this treatment, the volume occupied by the powder was reduced to 15% of its original value and the powder material was transformed into monolithic pieces. This shrinking effect is attributed to the diminished electrostatic repulsion which exists between dehydroxylated TiO2 nanocrystals in the absence of a liquid. A microstructural analysis was carried out by transmission electron microscopy (TEM), and representative TEM images of TiO2 nanocrystals before and after water exposure (plus dehydration at 473 K) are shown in parts a and b of Figure 1, respectively. The particle size distributions were determined from different TEM images and indicate that with 13 nm the average particle size has not been affected by water treatment. Electron diffraction and X-ray diffraction confirm anatase crystal structure for both cases. Furthermore, X-ray diffraction revealed no change in the reflection widths between the dry and watertreated samples. Consistent with the TEM observation, an average particle diameter of 13 nm was confirmed by using the Scherrer formula. Despite these identical particle properties, one can clearly conclude from the micrograph in Figure 1b that the water-treated particles are much more agglomerated. Consequently, the average distance between the individual grains must be significantly reduced compared with the original material (Figure 1a). © 2006 American Chemical Society
7606 J. Phys. Chem. B, Vol. 110, No. 15, 2006
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Figure 1. Typical transmission electron micrographs of (a) TiO2 nanoparticles obtained by MOCVD and subjected to extensive oxygen treatment afterward and (b) the same material after contact with water and subsequent drying at 200 °C and p < 10-5 mbar. The images were taken with a 200 kV field emission gun microscope (Philipps Tecnai F20).
Figure 3. Schematized condensation effect of originally dry TiO2 nanocrystals (a) that form a mesoporous structure (d) after watermediated particle agglomeration (b and c).
been formed, as illustrated by the scheme in Figure 3. The average size and the crystallographic structure of the nanocrystals, however, have remained unaltered. Removal of lattice oxygen by thermal treatment under vacuum conditions leads to oxygen vacancy formation and electronic reduction23 of the nanocrystals according to
2O2- f O2 v + 4eFigure 2. Nitrogen sorption isotherms (a) of dry and (b) of watertreated TiO2 nanocrystals. The inset shows the BJH pore diameter distributions of the two samples.
The nitrogen adsorption-desorption isotherms of the materials before and after water exposure (parts a and b of Figure 2, respectively) have been obtained to determine surface area19 and porosity.20 According to the IUPAC classification system,21 the isotherms of sample a (before water treatment) can be regarded as type II. The absence of a final saturation plateau points to macropores not detectable by nitrogen sorption up to 1 bar. The small hysteresis loop is associated with capillary condensation in mesopores and small macropores. However, the isotherms of the water-treated sample (Figure 2b) are of type IV. They exhibit a final saturation plateau and a hysteresis loop which is indicative of a mesoporous material with a uniform mesopore size. Pore size distributions of the two materials have been determined using the Barret, Joyner, and Halenda (BJH) model.20 Before H2O treatment, the sample of TiO2 nanocrystals (Figure 2a) reveals a broad pore size distribution which reaches beyond the pore diameter range not detectable by nitrogen sorption and has a maximum at 32 nm. Water exposure leads to a decrease of both, the mean pore diameter as well as the pore size distribution. A value of 16 nm for the mean pore diameter is about the same as the average nanocrystal size.22 Identical values for the specific surface area SBET ) 130 ( 13 m2‚g-1 were obtained for both materials, and therefore, significant sintering and loss of surface area as a consequence of water treatment may be excluded. In agreement with the enhanced particle agglomeration which is suggested by the significant reduction in powder volume as well as transmission electron microscopy (Figure 1), the profound change in the adsorption-desorption hysteresis (Figure 2) indicates that water exposure transforms the loose agglomerate of nanocrystals into an interconnected mesoporous particle network with a uniform pore size. Consequently, it has to be expected that interfaces between adjoined nanocrystals have
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The electrons originating from the removed oxygen anions are either localized at regular or interstitial cations to form paramagnetic Ti3+ states17,24 or produce, as delocalized species, a continuous absorption in the middle infrared range.16 When molecular oxygen is added to such samples, interfacial electron transfer occurs and adsorbed O2- radicals, which are perfectly detectable with electron paramagnetic resonance spectroscopy, are observed.25 Extensive oxygen treatment of the MOCVD powder at 870 K was applied to obtain dry and oxidized anatase nanocrystals as the starting material of this study. This procedure guarantees that, with respect to thermally induced reduction, the material remains under high vacuum conditions stable up to 900 K. However, after being subjected to a hydration-dehydration cycle, lattice oxygen extraction from these nanocrystals is significantly facilitated: heating to T ) 670 K at p < 10-5 mbar already generates EPR signatures consistent with electronically reduced Ti3+ species in TiO2 nanocrystals. Annealing to higher temperatures such as T ) 870 K (Figure 4a) enhances this effect. The respective spectrum contains signals specific to Ti3+ centers in anatase TiO2 as well as an isotropic feature at g ) 2.0025 (indicated by an arrow) which, after identical treatment, is absent on TiO2 nanocrystals which had not been subjected to the water treatment previously. Oxygen addition at a pressure of 10 mbar perfectly bleaches the Ti3+ signal, whereas the sharp feature at g ) 2.0025 (Figure 4b, indicated by an arrow) gains intensity compared to the vacuum annealed sample (Figure 4a, indicated by an arrow). After pumping down to 10-6 mbar (Figure 4c), an intense EPR signal characteristic of adsorbed O2- species appears, whichsdue to spin exchange interaction between the adsorbed surface radicals and gas-phase O2 (10 mbar)sis absent in Figure 4b. Addition of SF6 with a higher electron affinity (1.05 eV versus 0.45 eV for O2) enhances the signal intensity at g ) 2.0025 by more than a factor of 10. Interfacial electron transfer between the solid and SF6 does not occur. Infrared spectroscopic experiments comparable to those in EPR were carried out in order to address the effect of water
Letters
Figure 4. Electron paramagnetic resonance signals on vacuum annealed TiO2 nanocrystals, which had been previously exposed to H2O (a). Addition of oxygen and EPR measurement in O2 presence reveal a paramagnetic center at g ) 2.0025 (b). Subsequent evacuation produces an intense signal consistent with adsorbed O2- ions (c).
exposure on the surface structure. The surface coverage with hydroxyls was monitored via their OH stretching bands. It was observed that water admission via the gas phase gave rise to essentially the same result as that obtained after sample contact with liquid water (not shown). Moreover, it was found that with increasing temperature of annealing at p < 10-5 mbar the OH coverage decreases. After degassing at T ) 870 K, when the EPR signal at g ) 2.0025 has its maximum (Figure 4c), the intensity and position of OH bands are identical to those measured on oxidized TiO2 nanocrystals before H2O admission. It is therefore unlikely that the electron center signal is a result of a hydroxylated and thus structurally altered surface. In oxide semiconductors, the formation of oxygen vacancies26 is associated with the respective reduction enthalpy of the solid. As compared to conventional coarse-grained materials, these values can be significantly reduced for nanocrystalline materials.27,28 This is attributed to smaller defect formation energies at grain boundaries, that is, interfaces between adjoined crystals, the density of which is clearly enhanced for nanocrystalline grains.28 In the present case, significant interparticle contact can be expected for the water-treated sample on the basis of the N2 sorption data (Figure 2b). The solvent induced decrease of the average interparticle distance (Figure 3c) finally leads to particle attachment and possibly the formation of chemical bonds between different grains (Figure 3d). Consequently, the adjoined nanocrystals have to share interfaces29 which comprise new sites associated with decreased defect formation energies.28 For isolated TiO2 nanocrystals which essentially lack such interfaces, thermal treatment at considerably higher temperatures (T > 900 K) has to be applied in order to generate oxygen vacancies. As an additional or alternative mechanism for vacancy formation, H2O adsorption followed by the desorption of neutral species, such as OH radicals or H2O2 molecules, can also account for the facilitated removal of lattice oxygen in the case of the watertreated samples. Besides porosity and reduced defect formation energies, the EPR signal at g ) 2.0025 represents a significant change in the electronic properties as observed for isolated TiO2 nanocrystals. Such paramagnetic electron centers which do not react chemically with O2 have been reported for TiO2-based materials in the past. Serwicka et al.30 found that on oxygen deficient TiO2 the adsorption of electron acceptors such as O2, SO2, or SF6 leads to an intensity increase of a symmetrical signal at g ) 2.003. This effect has been attributed to conduction band electrons localized at anion vacancy sites, where the activation
J. Phys. Chem. B, Vol. 110, No. 15, 2006 7607 energy for the localization process was estimated to be around 4 kJ‚mol-1. In addition to diverse other reports,31,32 a recent investigation revealed a clear correlation between the concentration of this type of center and photocatalytic activity in the visible light range.33 Furthermore, the observation of polarizable electron centers in TiO2 has some analogy to findings on mesoporous silica: by means of EPR spectroscopy, it was shown by Chiesa et al.34,35 that the adsorption of Lewis bases causes a dramatic increase of the electron population in the conduction band. In the present investigation, the simultaneous occurrence of interparticle contacts induced by water treatment, on one hand, and the paramagnetic center which resonates at g ) 2.0025 and is absent on isolated grains, on the other hand, is notable. It is speculated that intergranular contacts and thus interparticle conduction take effect in this mesoporous sample and electron delocalization over more than one nanocrystal can occur. Clearly, a more detailed investigation of this phenomenon which includes a thorough characterization of the electron center is required and currently under way. In conclusion, we report the water-mediated reduction of the interparticle distance of TiO2 nanocrystals. From sorption experiments and microstructural TEM analysis, we conclude intergranular connection between originally isolated nanocrystals. Altered particle properties, such as facilitated reducibility induced by vacuum annealing and the generation of electron centers that are susceptible to the adsorption of gases, were measured. In light of the material transformation discussed, the generation of new electronic states and extended charge carrier delocalization is interesting for a variety of technological applications. For example, it directly connects to photochemical energy conversion where TiO2 nanoparticles serve as electron conductors and intergranular electron transfer during operation is required. Furthermore, oxide systems used in gas sensor devices are based on the reversible interaction between charge carriers and adsorbed gases. The role of grain boundaries in these devices cannot be underestimated.36 Acknowledgment. This work was supported by the Austrian Fonds zur Fo¨rderung der Wissenschaftlichen Forschung (FWF -P17514-N11). We would like to thank Magdalena Janus and Markus Handler for their assistance with some of the experiments as well as Mario Chiesa, Elio Giamello, and Peter V. Sushko for discussions and their valuable comments. Supporting Information Available: Details about the TiO2 nanoparticle production, the N2 sorption experiments, and the spectroscopic measurements as well as a statement about possible carbon contamination. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Jun, Y.-W.; Lee, J.-H.; Choi, J.-S.; Choi, J. J. Phys. Chem. B 2005, 109, 14795. (3) Stankic, S.; Mu¨ller, M.; Sterrer, M.; Bernardi, J.; Diwald, O.; Kno¨zinger, E. Angew. Chem., Int. Ed. 2005, 44, 4917. (4) Barnard, A. S.; Curtiss, L. A. Nano Lett. 2005, 5, 1261. (5) Zhang, H. Z.; Gilbert, B.; Huang, F.; Banfield, J. F. Nature 2003, 424, 1025. (6) Zhang, H. Z.; Banfield, J. F. Nano Lett. 2004, 4, 713. (7) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (8) Watson, D. F.; Meyer, G. J. Annu. ReV. Phys. Chem. 2005, 56, 119. (9) Bisquert, J.; Cahen, D.; Hodes, G.; Ru¨hle, S.; Zaban, A. J. Phys. Chem. B 2004, 108, 8106. (10) Thompson, T. L.; Yates, J. T., Jr. Top. Catal. 2005, 35, 197.
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