See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/231956259 The Role of the Surface Coverage on the Structural and the Electronic Properties of TiO2 Nanocrystals Article in MRS Online Proceeding Library Archive · January 2009 DOI: 10.1557/PROC-1178-AA09-34 CITATIONS READS 0 18 6 authors, including: Giovanni Cantele Fabio Trani 77 PUBLICATIONS 1,604 CITATIONS 47 PUBLICATIONS 837 CITATIONS Italian National Research Council SEE PROFILE University of Naples Federico II SEE PROFILE Domenico Ninno Stefano Ossicini 137 PUBLICATIONS 2,489 CITATIONS 225 PUBLICATIONS 4,842 CITATIONS University of Naples Federico II SEE PROFILE All content following this page was uploaded by Stefano Ossicini on 18 June 2014. The user has requested enhancement of the downloaded file. Università degli Studi di Modena e Reggio Emilia SEE PROFILE Mater. Res. Soc. Symp. Proc. Vol. 1178 © 2009 Materials Research Society 1178-AA09-34 The Role of the Surface Coverage on the Structural and Electronic Properties of TiO2 Nanocrystals Amilcare Iacomino1, Giovanni Cantele2, Fabio Trani2, Domenico Ninno2, Ivan Marri3 and Stefano Ossicini3 1 Dipartimento di Fisica E. Amaldi, Università degli Studi Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy and CNISM, U. di R. Università degli Studi di Napoli Federico II. 2 CNR-INFM-Coherentia and Università degli Studi di Napoli Federico II, Dipartimento di Scienze Fisiche, Complesso Universitario Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy. 3 CNR-INFM-S3 and Università di Modena e Reggio Emilia, Dipartimento di Scienze e Metodi dell'Ingegneria, Via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia, Italy. ABSTRACT We present here a characterization of TiO2 0D nanoclusters and 1D nanowires in the framework of ab initio density functional theory (DFT) calculations. We analyze the effect of the surface coverage by functionalizing dangling bonds with simple adsorbates modeling the basical interactions of TiO2 nanosystems with the hydration sphere. We thus address the electronic reorganization and the surface role in determining the overall properties of the nanostructures. The structural reconstruction is found to depend on the surface coverage and the experimental evidences on the structural variations can be explained by a topological analysis of the Ti-O bonds. Q-size effects are observed through the bandgap widening, but the surface competes to determine the energy distribution of the electronic levels. The hydrogenated nanocrystals do show occupied levels at the bottom of the conduction bands, which can enhance the conductive properties of the nanowires. In the hydrogenated cluster such levels present a localized charge distribution with strong similarities (orbital character, energy position) to the defect states arising after oxygens desorption. From the analysis of the electronic density of states we found that Ti-H bonds induce in-gap states above the valence bands, whereas hydration leads to occupied states that shift the valence bands to lower binding energies. INTRODUCTION The possibility to enhance the sunlight conversion efficiency in the modern generation of metal oxides based nanodevices is a key issue for the nowadays research and of social relevance for a sustainable energy production. TiO2 based dye-sensitized solar cells (DSSC) are emerging for the contained costs, versatility, increasing stability through commercial standards and fairly good efficiencies [1,2]. Nanostructured TiO2 also shows higher photocatalytic efficiencies for several heterogeneous reactions such as pollutant compounds decomposition and water splitting [3]. All these applications are based on the possibility to enhance the photogenerated charge separation at the interface between the nanocrystal and the sensitizing molecule in DSSC and on the efficient charge transfer from/to the reactive species for the photocatalysis. In the present study we want to highlight the role played by the surface configuration in determining the overall properties of thin TiO2 nanostructures in both its zero- and one-dimensional forms. Keeping in mind that surface coverage due to water or solution species are always present on the surface of the nanocrystals, we do show the effects of hydrogenation and hydration of the surface undercoordinated atoms on the structural and electronic properties of these nanomaterials. THEORY All first-principles calculations are carried out within the DFT framework, employing the Quantum-ESPRESSO [4] based on plane waves and pseudopotentials. We used the generalized gradient approximation (GGA) parametrized with the Perdew-Wang exchange-correlation Functional [5] and Vanderbilt ultrasoft pseudopotentials [6]. The reciprocal space is sampled using a plane-wave basis set up to a kinetic energy cutoff of 30 Ry for the wave functions. The nanocrystals (NC) geometries are optimized with the BFGS direct energy minimization technique. All properties presented are referred to the optimized systems. A vacuum gap of at least 6 Å separates each NC from its periodic replica. The model structures are initially defined in the ideal bulk position of the anatase crystal phase. The bare stoichiometric cluster is made of 87 atoms with chemical formula (TiO2)29. Its morphology derives from a perfect bipyramid with [101] and [011] lateral surfaces, then truncated along the [001] direction[7]. By linking in chain along the [001] direction different replica of this cluster we can define the thinnest stoichiometric nanowire (NW) and then consider NWs of the same morphology and with larger diameters to have an estimate of the dependecy of the properties on the size. Hydrogenation is defined by bonding each undercoordinated atom of the surface with a H atom and hydration is defined by bonding each undercoordinated O of the surface with a H atom and each undercoordinated Ti with a OH group, to mimic an overall dissociative adsorption of water. DISCUSSION In Figure 1 the Ti-O distance distribution is shown. There is an evident dispersion of length distances around the ideal bulk values (dotted vertical lines) which is expected for spatially confined nanostructures. However the covered NCs present peaks corresponding to the bulk distances which are clearly distinguishable with respect to the moltitude of peaks of the bare NC. We deduce that the surface coverage stabilizes a better crystalline organization with respect to the naked NC. Experimentally, a decrease of the coordination number and a broadened distribution of their values are observed [8]. The Ti-O first shell tends toward contraction in all of the NCs, in agreement with experimental findings [9]. Experimental samples show a reconstructed octahedron with three short and three long bonds [9], whereas for the bulk anatase 4 short and 2 long bonds are found. We then considered as Ti-O first shell all the bonds shorter than 1.95 Å, hence the longer bonds as Ti-O second shell. We find that the bare NCs are the closest to the bulk ratio whereas hydrated NCs are the closest to experimental samples. To explain the two main experimental findings, i.e. the decrease of the first shell Ti-O bond and the match between the first and second shell coordination number, we decomposed the whole distance distribution in the contributions due to different atomic pairs. From this analysis on hydrated NCs we find that bulk oxygens (3-fold coordinated) faithfully reproduce the 2:1 ratio of the distances in the bulk octahedron, thus other oxygens are the source of experimental variations. We find that the increase of the second shell coordination number is entirely caused by surface (H-bonded) oxygens. Then oxygens coming from covering hydroxyl groups Figure 1. Density of Ti-O distancies distribution for the thinnest bare (bottom), hydrogenated (middle), and hydrated NWs (top panel). Dotted vertical lines at the bulk values (with coordination numbers shown on the top). Boxes show experimental variations. The total density of distancies is decomposed in its local contributions as indicated in the legend. the first peak at the shortest bonds. We thus state that OH radicals, adsorbed on the surface of NCs, are the main source of the first shell contraction actually observed. On the other hand, the experimental evidence of the 1:1 ratio of the first two shells can be entirely ascribed to chemisorption of OH groups which can fill the oxygen vacancies in the synthesis process. Thus the two main experimental variations are due to two different sources, that is the adsorption of two kinds of OH groups on the NCs surface. At last, isolated and then highly reactive oxygens, possibly present on the NC surface, may produce the shortest bonds as shown by the bare cluster. Electronic properties In Figure 2 we show a projection of the density of states (DOS) on the atomic orbitals. The third group of levels [figure 2 (a)], is strongly affected by the surface bonds. For both coverages, the levels mainly related to H-bonded oxygens (group of peaks A, C) are all located at higher binding energies than those related to unsaturated oxygens (group of peaks B, D). At variance, in the bare NCs, the levels related to undercoordinated surface oxygens are all located at lower binding energies. Thus the surface coverage induces a level shift of these O 2s orbitals to higher binding energies than those related to bulk-like oxygens. For the lower energy valence levels, figure 2 (b), both type of coverages do show a subgroup of levels with higher binding energies than those related to the (bulk-like) Ti-O bonds. Such subgroups are labelled E and G and are ascribed to the O-H chemical bond. Hydrogen coverage produces an occupied subgroup of states in the forbidden bulk gap, labelled F in Figure 2 (b), just above the (bulk-like) Ti-O Figure 2. Density of states (DOS) of the valence states for the bare (bottom), hydrogenated (middle) and hydrated clusters (top panels), projected on the O orbitals. Ef is the Fermi level. valence band (VB) states, whereas hydration results in a significant increase of the DOS below the (surface) O 2p valence energies. The subgroup F is substantially related to the Ti3d-4s-H1s chemical bond, absent in the bare and hydrated NCs. Therefore, the reduction of Ti atoms promoted by the formation of Ti-H bonds leads to the opposite effect with respect to the Ti-OH bond: occupied states show up at lower binding energy than the typical top valence states of TiO2. This effect can be a source of a possible red shift of the band gap, eventually hiding the blue shift caused by the Q-size effect. Our results show that Ti-OH bonds do not induce states in the gap but they stabilize states to higher binding energies than the VB top. The intra-gap states do depend from possible Ti-H bonds leading to occupied bands which narrow the band gap to less than 30 meV in the hydrogenated NWs. In the same way the progressive desorption of the one-coordinated oxygens from the [001] surface of the bare cluster leads to NCs with an energy gap of about 0.2 eV and an increasing content of occupied states near the CB. The blue shift obtained for the bare NCs (0.25 eV for the cluster) is enhanced in the hydrated NCs (0.70 eV for the hydrated cluster). We expect that the Q-size effect can be detectable not only in naked NCs but also in water covered NCs. We thus observe a dependence on the blue shift from the different structural organization induced by the surface coverage. The nature of the electronic orbitals Bare and hydrated NCs do have valence and conduction states whose spatial charge distribution resembles the analogous states of the bulk anatase, i.e. O 2p and Ti 3d orbital characters respectively. More interestingly hydrogenated NCs present occupied states with an exclusive Ti 3d character whose spatial distribution is localized in a restricted region for the cluster whereas it is delocalized on the whole structure for NWs. The hydrogenated cluster HOMO is a defect state whose energy is just below the conductive empty states that can be linked to the experimental observations obtained in the dark, at low temperature, and exposure to hydrogen gas [10]. The same Ti 3d orbital character, charge spatial distribution and energy range below the empty states can be obtained by desorption of one coordinated oxygens on the surface of the bare cluster. Thus hydrogenation and oxygen deficiency can induce analogous defects. The highest occupied bands of hydrogenated NWs are energetically close to conduction bands, reducing the band gap to less than 30 meV. The corresponding electrons can be easly promoted to the CB thus leading to an enhancement of the conductive properties of hydrogen covered NWs. An increase of the conduction is actually found for TiO2 nanotubes exposed to hydrogen gas [11]. The authors deduce that the most likely process of conductivity increase is linked with the dissociative adsorption of hydrogen on the surface of the nanotubes. CONCLUSIONS We performed DFT calculations on 0D and 1D TiO2 NCs with different surface coverages, modelling different synthesis conditions. The hydrated NCs are the best models since they are able to reproduce the observed 1:1 ratio of the first and second shell coordination numbers and the contraction of the first shell Ti-O bond length. The source of these variations with respect to the bulk anatase have been identified in topologically different OH groups on the surface of the NCs. All covered NCs are more structurally ordered with respect to the bare counterpart. The Q-size effects are linked to the surface coverage, since terminal atoms can influence the structural order thus leading to different amount of band gap shift for NCs of the same size. Hydrogenation can lead to occupied states close in energy to conduction bands, thus improving the conductive properties of 1D NWs as the charge distribution of such states is found to be delocalized on the whole structure. Hydration of NCs better stabilizes the models thus stressing the important role of the surface termination in determining the photocatalytic performance of confined nanostructures [12]. ACKNOWLEDGMENTS We acknowledge the support of the MIUR PRIN Italy. All the calculations were performed at CINECA-Bologna ("Iniziativa Calcolo Parallelo del CNR- INFM"), and ``Campus Computational Grid''-Università di Napoli Federico II. A. I. acknowledges CNISM for financial support. REFERENCES 1. 2. 3. 4. X. Chen and S. S. Mao, Chem. Rev. 107, 2891 (2007). M. Grätzel, Nature 414, 338 (2001). O. 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