PCCP View Article Online
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
PAPER
Cite this: DOI: 10.1039/c4cp02789b
View Journal
A DFT study of the effect of OH groups on the optical, electronic, and structural properties of TiO2 nanoparticles Olga Miroshnichenko,†a Sami Auvinen*b and Matti Alataloa The effects of on-surface OH groups on the structural and optical properties of small TiO2 particles have been studied in order to obtain knowledge about the optical behaviour of the TiO2 nanoparticles in solutions. The standard density functional theory was used to model the structural changes, and time-
Received 25th June 2014, Accepted 9th January 2015
dependent density functional theory was used to address the changes in the photoabsorption characteristics of an anatase-structured (TiO2)16 cluster. It was shown that the OH groups can alter both
DOI: 10.1039/c4cp02789b
the geometric and electronic structure of the clusters, resulting in changes in the optical properties. The
www.rsc.org/pccp
shown to be reduced by OH adsorption.
large blue shift, obtained in earlier calculations for TiO2 nanoparticles as compared with bulk TiO2, is
1 Introduction Titanium dioxide (TiO2) is one of the most investigated and most widely used metal oxides. Such popularity comes from the broad variety of applications of this wide band gap semiconductor in an enormous number of fields of industry and technology. These applications can be classified into energy and environmental applications. Titanium dioxide is used in solar cells,1–3 photocatalytic splitting of water under ultraviolet (UV) light,4–6 removal of organic pollutants, and in photo- and electrochromics.7–10 Its biological and chemical inertness and affordability explain the abundant use of TiO2 as a pigment11 in paints and food dyes.12 Its material properties depend strongly on the manufacturing method of the particles and on their geometrical and electronic structure.13–16 Relatively large particles with a size of approximately 200 nm are opaque and extremely white, which explains the usage of TiO2 as a pigment. When their size is decreased down to 10 nm, the particles become transparent, while still maintaining their ability to absorb UV-radiation, which makes them useful in sunscreens and UV-protectors.17,18 One of the main manufacturing methods of TiO2 nanoparticles is based on hydrolysis from various solutions or heating in water. Moreover, many applications and experiments with TiO2 take place in aqueous environments.19–21 Studies of water adsorption and therefore the adsorption of hydroxyl groups (OH) a
Department of Physics, University of Oulu, P.O. Box 3000, FI-90014, Oulu, Finland Faculty of Technology, Department of Mathematics and Physics, Lappeenranta University of Technology, P.O. Box 20, FI-53851, Lappeenranta, Finland. E-mail: [email protected] † Also at: Department of Theoretical Mechanics, St. Petersburg State Polytechnical University, Polytechnicheskaya 29, 195251, St. Petersburg, Russian Federation b
This journal is © the Owner Societies 2015
are thus highly important. A lot of experimental and theoretical studies on the adsorption of water22–29 and other molecules on titanium dioxide surfaces have been conducted. Bredow and Jug30 showed that the dissociative adsorption of water on rutile and anatase surfaces is more stable than molecular adsorption, and, moreover, molecularly adsorbed water is not active in photoreactions. Dissociatively adsorbed water plays an important role in the first steps of the photocatalytic degradation of organic water contaminants.31 In the early stages of precipitation, OH groups also take part in TiO2 structure formation and particle growth.32 Other studies have shown a big influence of adsorption on the optical properties of the material.33–36 For example, H adsorption causes a defect state 0.94 eV below the bottom of the conduction band.37 It was also shown that the doping or sensitizing of titanium dioxide extends its photoactivity to the visible light region.38,39 We have shown in our earlier work40,41 that the geometric structure of the clusters plays an important role in determining the optical properties. It is therefore necessary to use computational methods which address the geometric, electronic and optical properties simultaneously. Such an approach is provided by density functional theory (DFT), which, together with time-dependent DFT (TDDFT) can describe these properties with great accuracy. In the present work, we study the effect of the OH coverage on a (TiO2)16 nanocluster by calculating the geometric, electronic, and optical properties for the structure covered by one, three, six, and sixteen OH groups. This particular cluster was chosen since it is sufficiently small to allow a set of calculations to be performed in a reasonable time, yet large enough to be non-trivial. In our previous studies we have performed calculations comparing TiO2 nanoparticle structures
Phys. Chem. Chem. Phys.
View Article Online
Paper
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
which are as symmetrical as possible and more needle-like ones, and in all tested cases, more symmetrical structures were energetically more favorable.40 Based on these results, and results by Persson et al.,42 we selected a (TiO2)16 cluster for our test case. The shape, size, and structure of the selected cluster provides us with a reliable base for studies since it is large enough to have a clear anatase structure, it is stoichiometric, and the bonding environment is reasonably defect free with regard to the number of dangling surface bonds.42
2 Computational details All structural and electronic calculations in this study have been performed using the GPAW software package.43–45 GPAW is an efficient density functional theory based real space code using the projector-augmented wave (PAW) method.46,47 For the photoabsorption spectrum calculations we have used the TDDFT implementation48 in GPAW. All calculations were performed at zero Kelvin electronic temperature. The cluster structures were relaxed using a quasi-Newton minimizer so that all forces were smaller than 0.01 eV Å 1. The width of the Fermi-distribution was set to 25 meV in order to avoid the convergence problems related to some of the model structures. We used a grid spacing of 0.17 Å, and the computational cell was set up with non-periodic boundary conditions, with 7 Å of empty space surrounding the cluster. The cluster calculations were spin-paired using only the G-point. We used the conjugate gradient method for the eigensolver, and the Perdew, Burke, Ernzerhof (PBE)49 exchange correlation functional. All calculations were charge neutral. The density of states (DOS) plots for the clusters were obtained using basically the same computational settings as the structural optimization runs. In addition, the empty states were also required to be fully converged up to the ten highest bands with cluster calculations. The DOS were acquired with 2000 data points and a normalized Gaussian broadening of 0.1 eV. In order to further study the gap states found in the DOS results, we also performed atomic orbital projected DOS (PDOS) computations. The PDOS were acquired with the same computational settings as the DOS runs. When calculating the photoabsorption spectra we used the timepropagation TDDFT approach (TP-TDDFT). The spectra were calculated using a 16.0 attosecond time step with 1000 iterations, yielding a total simulation time of 16 femtoseconds. The grid-spacing was set to 0.3 Å, and the computational cell was set up with non-periodic boundary conditions, having 10 Å of empty space surrounding the cluster. The kick parameter for the initial distribution applied to the wave functions was 10 3. The spectra were calculated from the dipole moment files using a Gaussian broadening of 0.1 eV.
3 Results and discussion 3.1
Structural effects
In this work we address the effect of OH groups on the Ti16O32 structure, which was constructed so that all atoms should have
Phys. Chem. Chem. Phys.
PCCP
sufficient coordination. The relaxed structure is depicted in Fig. 1(a). We modelled the effect of one adsorbed OH group, along with three, six and 16 adsorbed OH groups on the cluster structure. The relaxed structures with adsorbed OH groups are presented in Fig. 1(b)–(e). Based on the results of Bredow and Jug30 we selected the on-surface titanium atoms as an adsorption sites for the OH-groups. The OH adsorption site in the case of one OH group was chosen using Avogadro molecular editor.50 The structures with hydroxyls adsorbed on all possible sites were test-relaxed with the built-in optimizer in Avogadro, using the Universal Force Field,51 which is appropriate for the full periodic table, including transition metals such as Ti. Five structures with minimal energies were relaxed in GPAW. The structure with the lowest energy became an input for electronic structure and optical property calculations in GPAW. Structures with 3 OH groups were tested only in Avogadro, due to the large amount of possible configurations and computational time limitations. With 6 OH groups the adsorption sites were chosen in order for the structure to be as symmetrical as possible. In the case of 16 OH groups, hydroxyls were attached to every Ti atom in the cluster. The structural data of the model particles has been collected in Tables 1–4. In the analysis of the bonds in the structures we have assumed that there is a Ti–O bond if the bonding distance is smaller than 2.2 Å, a Ti–Ti bond if the bonding distance is smaller than 2.94 Å, and an O–O bond if the bonding distance is smaller than 1.46 Å. Please note that in Tables 1–3 we have neglected the OH groups (bonds in OH groups and bonds between the cluster and OH groups) in order to see the structural changes in the underlying cluster structures; in Table 4 bonds between the cluster and OH groups are included in the analysis. The number of bonds in the structures after the relaxation are presented in Table 1. As we can see, the general trend is that the adsorption of OH groups tends to break or stretch Ti–Ti and Ti–O bonds in the structures, which is visible in Table 1 as a lower number of bonds in the relaxed structures with adsorbed hydroxyls relative to the number of bonds in the relaxed bare cluster. On the other hand, the average bond lengths in the remaining Ti–Ti and Ti–O bonds do not change dramatically, as we can see in Table 2. In structures with 3, 6 and 16 OH groups the total average length of all bonds seems to change more due to the appearance of one O–O bond in these structures. The relative percentage changes in three dimensions and average dimensions of the structures have been collected in Table 3. By looking at the data in Tables 1–3, we can conclude that the adsorbed OH groups reshape the TiO2 clusters by breaking or extending Ti–Ti and Ti–O bonds, so that the average dimension of the particles is increased. Regarding the coordination environment of the nanoparticles, the coordination data of the studied cluster structures is collected in Table 4. In the coordination analysis we have neglected the oxygen and hydrogen atoms in the on-surface OH groups, but the bonds between the surface titanium atoms and the oxygen atoms in the on-surface OH groups have been
This journal is © the Owner Societies 2015
View Article Online
Paper
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
PCCP
Fig. 1 The relaxed cluster structures: bare Ti16O32 (a), Ti16O32 with one OH group (b), Ti16O32 with three OH groups (c), Ti16O32 with six OH groups (d), and Ti16O32 with 16 OH groups (e). Oxygen atoms are marked with red color, titanium atoms with gray color, and hydrogen atoms are blue. The atoms corresponding to the observed defect states are numbered and emphasized with black rectangles. The red, green, and blue coordinate axes represent the x, y, and z axes correspondingly.
calculated for the coordination of the titanium atoms. This has been done in order to be able to see the effect of the adsorbed OH groups on the coordination of the underlying cluster structures. As a reference we have used the coordination of titanium and oxygen in bulk TiO2, where the coordination of titanium is
This journal is © the Owner Societies 2015
6-fold and the coordination of oxygen is 3-fold.52 Normally the coordination number of oxygen does not exceed 3, and if the coordination number is higher than three the bonding is ionic, whereas if the coordination number is under three the bonding is regarded as covalent.38
Phys. Chem. Chem. Phys.
View Article Online
Paper
PCCP
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
Table 1 Number of bonds in the relaxed structures. The OH groups are not taken into account
Structure
nTi–O
nTi–Ti
nO–O
nAll
Ti16O32 Ti16O32 Ti16O32 Ti16O32 Ti16O32
68 66 65 62 58
12 11 9 2 1
0 0 1 1 1
80 77 75 65 60
bare + OH + 3OH + 6OH + 16OH
Table 2 Average bond lengths in the relaxed structures. The OH groups are not taken into account
Structure
dTi–O/Å
dTi–Ti/Å
dO–O/Å
All/Å
Ti16O32 Ti16O32 Ti16O32 Ti16O32 Ti16O32
1.872 1.867 1.878 1.868 1.882
2.789 2.832 2.822 2.810 2.848
— — 1.443 1.452 1.334
2.009 2.005 1.985 1.891 1.889
bare + OH + 3OH + 6OH + 16OH
Table 3 Percentage changes in the dimensions of the relaxed clusters in three directions (x, y and z) relative to the relaxed bare TiO2 structure. Positive and negative values correspondingly stand for increases and decreases in the dimensions. The OH groups are not taken into account
Structure Ti16O32 Ti16O32 Ti16O32 Ti16O32
+ + + +
x OH 3OH 6OH 16OH
y
7.4 13.07 30.98 0.15
4.5 0.74 10.44 11.88
z
Avg.
14.5 6.37 12.26 31.83
4.8 5.26 7.36 20.07
Table 4 Coordination of Ti and O atoms in relaxed clusters. Atoms in the OH groups are not taken into account
Structure
3f-Ti
4f-Ti
5f-Ti
6f-Ti
7f-Ti
1f-O
2f-O
3f-O
Ti16O32 Ti16O32 Ti16O32 Ti16O32 Ti16O32
— 1 — — —
— 1 1 9 7
6 5 9 6 8
8 7 5 1 1
2 2 1 — —
— 1 — 2 5
28 28 29 28 26
4 3 3 2 1
bare + OH + 3OH + 6OH + 16OH
As we can see in the Table 4 the number of undercoordinated oxygen atoms varies within 3 bonds, and the number of under-coordinated titanium atoms within 9 bonds. The number of over-coordinated titanium atoms varies within 2 bonds. The total number of all under-coordinated atoms varies from 34 to 46, and the clusters with highest total number of under-coordinated atoms (structures with 6 and 16 adsorbed OH groups) are the ones with the highest photoabsorptions above 12.5 eV. The cluster with 46 under-coordinated atoms (cluster with 16 adsorbed hydroxyls) shows the highest photoabsorption also between 4 and 8.5 eV (Fig. 3). 3.2
Electronic properties
The DOS plots for the Ti16O32 structures with one, three, six, and sixteen adsorbed OH groups are presented in Fig. 2 along with the DOS of the bare structure for reference, and the band gap results are collected in Table 5. The DOS of bare Ti16O32 shows a clear band structure with a well defined valence band
Phys. Chem. Chem. Phys.
Fig. 2 DOS of bare Ti16O32 (a), Ti16O32 with one OH group (b), Ti16O32 with three OH groups (c), Ti16O32 with six OH groups (d), and Ti16O32 with 16 OH groups (e). HOMO and LUMO levels are plotted with vertical dashed lines. In (b), (c), and (e) HOMO and LUMO levels almost coincide.
(VB) and conduction band (CB),‡ which corresponds well to the results by Persson et al.,42 who found that stoichiometric, adequately coordinated TiO2 clusters will have well defined, defect-free band gaps. Adding one OH group introduces empty states just above the edge of the VB, resulting in an almost negligible difference between the HOMO and LUMO levels (DHOMO–LUMO). This is interesting since the fundamental band gap (separation between VB and CB) still remains as large as 2.947 eV, which is actually almost 0.114 eV larger than the DHOMO–LUMO of the bare cluster. The PDOS results reveal that the empty state above the valence band is an Op state of the oxygen atom 1 in Fig. 1(b). The adsorbed OH group alters the cluster structure during the relaxation so that oxygen atom 1 has been separated from one titanium atom. This results in a terminal Ti–O bond, which is energetically unfavourable, and the breaking of the bulk symmetry causes the empty Op state to appear at the edge of the valence band. ‡ Please note, that here we use the notation ‘‘band’’, but in fact the states are more localized than continuous in the case of very small clusters.
This journal is © the Owner Societies 2015
View Article Online
PCCP
Paper
Table 5 Electronic structure information for the Ti16O32 clusters (all energies are in eV). All structures are relaxed
Structure Ti16O32 Ti16O32 Ti16O32 Ti16O32 Ti16O32
+ + + +
OH 3OH 6OH 16OH
HOMO
LUMO
0.626 0.033 0.121 0.884 0.014
2.207 0.024 0.001 1.460 0.006
DHOMO–LUMO
Eg
2.833 0.009 0.120 2.344 0.020
2.833 2.947a 2.875a 2.344 2.360a
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
a The gap (Eg) was calculated as the difference between the edges of the valence and conduction bands.
The adsorption of three OH groups causes the appearance of empty states just below the Fermi level; these states are Op states of the oxygen atoms 2 and 3 in Fig. 1(c), caused probably by changes in the coordination of the surrounding titanium atoms during the relaxation. In the case of six adsorbed OH groups there are clear empty gap states around 1.5 eV, which lowers DHOMO–LUMO. PDOS results show that these empty states are d states of titanium atom 4 in Fig. 1(d). These states are due to the lower coordination of titanium atom 4 after the relaxation, and due to the exceptional triple O coordination environment (oxygen atoms 5, 6, and 7 in Fig. 1(d)) next to it. In the case of 16 adsorbed OH groups, the almost negligible DHOMO–LUMO is again due to the empty Op states of the oxygen atoms 8 and 9 in Fig. 1(e). Here the empty states are located at the edge of the valence band due to the O–O bond of oxygen atoms 8 and 9. Adding more OH groups on the cluster surface clearly shifts the intensity of the DOS so that the VB states become more localized at the lower edge of the VB (from 8.5 to 4 eV), and the CB states become more localized at the upper edge of the CB (from 6 to 9.5 eV). The appearance of new states at the lower part of VB is due to the contribution of the oxygen atoms in the OH groups. The increase in the intensity of the DOS in the upper edge of the CB is caused by the increase in the number of atoms in the system; in this energy region all atoms contribute to the DOS without the explicit prevalence of particular atoms. In the case of one, three, and sixteen adsorbed OH groups the DHOMO–LUMO value of the system is almost negligible, but in the case of six adsorbed OH groups, the DHOMO–LUMO value is 2.34 eV. The symmetry of the structures can affect the electronic structure so that values of DHOMO–LUMO decrease. Structures with one and three adsorbed hydroxyls are not symmetrical, which results in an almost zero DHOMO–LUMO value. The structure with 16 OH groups undergoes strong rearrangment during the structure relaxation, inducing defects in the structure. These defects may be responsible for the lowering of the DHOMO–LUMO value. In the case of the Ti16O32 structure, we can use the computed and estimated band gaps, and the average is 2.67 eV. When this value is compared to our earlier computational result for the band gap of bulk anatase (2.12 eV),40 we see that there is an average gap broadening of 0.55 eV, which corresponds well with previously reported values of 0.1–0.6 eV.20,53,54 3.3
Fig. 3 Total averaged photoabsorption spectra for the bare Ti16O32 structure, and the Ti16O32 structure with one OH group, with 3 OH groups, with 6 OH groups, and with 16 OH groups.
clearly visible that the trend observed in the DOS results shows up also in the photoabsorption characteristics of the clusters. The on-surface OH groups enhance the absorption at lower energies (7–9 eV) and at higher energies (12.5–30 eV). One adsorbed OH group does not significantly alter the photoabsorption spectrum, although small enhancements at the above mentioned energies can be observed. Three and six adsorbed OH groups clearly alter the photoabsorption characteristics, and in the case of 16 adsorbed OH groups the change is already significant. In the inset of Fig. 3 the magnified spectra curves in the low energy region are presented. It can be seen that for structures with small differences between HOMO and LUMO, photoabsorption starts already at very low energy. However, since the density of empty states near the VB edge is low, the effect of the low energy transitions does not significantly alter the absorption threshold at larger scale. The total averaged (averaged over all coordinate axes) refractive index functions (RIFs) for the same structures are presented in Fig. 4. The derivation of RIFs from photoabsorption spectra is presented in ref. 41. What should be noted here is
Optical properties
The total averaged (averaged over all coordinate axes) photoabsorption spectra for all structures are presented in Fig. 3. It is
This journal is © the Owner Societies 2015
Fig. 4 Total averaged refractive index functions for the bare Ti16O32 structure, and the Ti16O32 structure with one OH group, with three OH groups, with six OH groups, and with 16 OH groups.
Phys. Chem. Chem. Phys.
View Article Online
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
Paper
that when calculating the RIF from the photoabsorption spectrum we suppose that the particle is purely anatase structured TiO2, and neglect the effect of OH groups on the density and valence electron density of the particle. However, this only affects the amplitude and level of the RIF, and the peaks and shape of the RIF remain unaffected. The effect of the OH groups on the dimension of the particles is included in the calculation of RIFs. The trend observed in the photoabsorption characteristics of the selected particles is also visible in the RIFs. The on-surface OH groups change the RIF at longer wavelengths (130–350 nm) and at shorter wavelengths (50–100 nm). With one adsorbed OH group small enhancements at the above-mentioned wavelengths can be observed, and with 16 adsorbed OH groups the changes are clearly visible. Previously we have reported significant blue shifts when the computational RIFs of the TiO2 nanoparticles were compared to the experimental bulk values.41 The present results for the photoabsorptions and RIFs of the Ti16O32(OH)n structures will complement our previous results, and show that the blue shifts in the case of the TiO2 clusters can be smaller when the clusters are covered with adsorbants, or are in aqueous solutions. Moreover, the results clearly reveal the fact that when considering the computationally modelled absorption properties of the TiO2 nanoparticles, the effects of the surrounding media will have to be taken into consideration. For example for the refractive index, the usual procedure to estimate the effect of the surrounding media for bare TiO2 nanoparticles ignores the effects of particle–adsorbate interaction.
4 Conclusions We have studied the effects of adsorbed OH groups on the atomic and electronic structures, photoabsorption spectra, and RIFs of Ti16O32 nanoparticles. The results show that adsorbed OH groups tend to break or stretch some near-surface Ti–Ti and Ti–O bonds in the underlying structures, while on the other hand the average bonding lengths of the remaining bonds do not change dramatically. The adsorbed OH groups cause an enhancement of the intensity of the DOS of the nanoparticle at the lower part of the valence band around 4 eV, whereas the intensity of the DOS of the conduction band is enhanced at the upper part of the band around 7 eV. The same trend is observed in the photoabsorption spectra of the Ti16O32 nanoparticles, where the absorption is increased at lower (7–9 eV) and higher energies (12–30 eV). The trend observed in the DOS and photoabsorption characteristics is also visible in the RIFs of the particles. The results indicate that the effects of the surrounding media have to be taken into consideration when considering the computationally modelled absorption properties of small TiO2 nanoparticles.
PCCP
Technology, and Sachtleben Pigments Oy,§ Pori, Finland for funding this research. We would also like to acknowledge Juho¨ki, Pertti Jalava, PhD from Prof. Math Oy, Ralf-Johan Lamminma PhD, and Minna Kuusisto, MSc, from Sachtleben Pigments Oy,§ and Dr Erik Vartiainen from Lappeenranta University of Technology for their comments. The computational resources were provided by CSC-Scientific Computing Ltd, Espoo, Finland.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
¨tzel, Prog. Photovoltaics, 2000, 8, 171–185. M. Gra ¨tzel, J. Photochem. Photobiol., C, 2003, 4, 145–153. M. Gra C. G. Granqvist, Adv. Mater., 2003, 15, 1789–1803. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38. A. Fujishima, T. N. Rao and D. A. Tryk, J. Photochem. Photobiol., C, 2000, 1, 1–21. D. A. Tryk, A. Fujishima and K. Honda, Electrochim. Acta, 2000, 45, 2363–2376. ¨tzel, Nature, 2001, 414, 338–344. M. Gra ¨tzel, Chem. Rev., 1995, 95, 49–68. A. Hagfeldt and M. Gra A. L. Linsebigler, G. Lu and J. J. T. Yates, Chem. Rev., 1995, 95, 735–758. A. Mills and S. L. Hunte, J. Photochem. Photobiol., A, 1997, 108, 1–35. G. Pfaff and P. Reynders, Chem. Rev., 1999, 99, 1963–1981. J. H. Braun, A. Baidins and R. E. Marganski, Prog. Org. Coat., 1992, 20, 105–138. A. P. Alivisatos, J. Phys. Chem., 1996, 100, 13226–13239. A. P. Alivisatos, Science, 1996, 271, 933–937. C. Burda, X. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev., 2005, 105, 1025–1102. Y. Hwu, Y. D. Yao, N. F. Cheng, C. Y. Tung and H. M. Lin, Nanostruct. Mater., 1997, 9, 355–358. A. Salvador, M. C. Pascual-Marti, J. R. Adell, A. Requeni and J. G. March, J. Pharm. Biomed. Anal., 2000, 22, 301–306. R. Zallen and M. P. Moret, Solid State Commun., 2006, 137, 154–157. A. Henglein, Top. Curr. Chem., 1988, 143, 113–180. C. Kormann, D. W. Bahnemann and M. Hoffmann, J. Phys. Chem., 1988, 92, 5196–5201. ¨tzel, J. Am. Chem. D. Duonghong, E. Borgarello and M. Gra Soc., 1981, 103, 4685–4690. R. L. Kurtz, R. Stockbauer, T. E. Madey, E. Roman and J. L. de Segovia, Surf. Sci., 1989, 218, 178–200. M. J. Jaycock and J. C. R. Waldsax, J. Chem. Soc., Faraday Trans. 1, 1974, 70, 1501–1517. M. Tsukada, H. Adachi and C. Satoko, Prog. Surf. Sci., 1983, 14, 113–174. J. Goniakowski, S. Bouette-Russo and C. Noguera, Surf. Sci., 1993, 284, 315. A. Fahmi and C. Minot, Surf. Sci., 1994, 304, 343. B. Boddenberg and W. Horstmann, Ber. Bunsenges. Phys. Chem., 1988, 92, 519.
Acknowledgements
26 27
We want to acknowledge the Finnish Academy of Science and Letters, the Research Foundation of Lappeenranta University of
§ Huntsman Pigments from 1.10.2014.
Phys. Chem. Chem. Phys.
This journal is © the Owner Societies 2015
View Article Online
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
PCCP
28 S. Bourgeois, F. Jomard and M. Perdereau, Surf. Sci., 1992, 279, 349–354. 29 Y. Suda and T. Morimoto, Langmuir, 1987, 3, 786–788. 30 T. Bredow and K. Jug, Surf. Sci., 1995, 327, 398–408. 31 C. S. Turchi and D. F. Ollis, J. Catal., 1990, 122, 178–192. 32 M. Gopal, W. J. M. Chan and L. C. D. Longhe, J. Mater. Sci., 1997, 32, 6001–6008. 33 K. L. Miller, C. B. Musgrave, J. L. Falconer and J. W. Medlin, J. Phys. Chem. C, 2011, 115, 2738–2749. 34 A. Iacomino, G. Cantele, D. Ninno, I. Marri and S. Ossicini, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 075405. 35 W. Zeng, T. Liu, Z. Wang, S. Tsukimoto, M. Saito and Y. Ikuhara, Mater. Trans., 2010, 51, 171–175. 36 C. O’Rourke and D. Bowler, J. Phys. Chem. C, 2010, 114, 20240–20248. 37 C. D. Valentin and G. Pacchioni, J. Phys. Chem. C, 2009, 113, 20543–20552. 38 Z. W. Qu and G. J. Kroes, J. Phys. Chem. B, 2006, 110, 8998–9007. 39 S. A. Shevlin and S. M. Woodley, J. Phys. Chem. C, 2010, 114, 17333–17343. 40 S. Auvinen, M. Alatalo, H. Haario, J.-P. Jalava and R.-J. ¨ki, J. Phys. Chem. C, 2011, 115, 8484–8493. Lamminma 41 S. Auvinen, M. Alatalo, H. Haario, J.-P. J. E. Vartiainen and ¨ki, J. Phys. Chem. C, 2013, 117, 3503–3512. R.-J. Lamminma 42 P. Persson, J. Gebhardt and S. Lunell, J. Phys. Chem. B, 2003, 107, 3336–3339. 43 J. J. Mortensen, L. B. Hansen and K. W. Jacobsen, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 71, 035109.
This journal is © the Owner Societies 2015
Paper
44 J. Enkovaara, C. Rostgaard, J. J. Mortensen, J. Chen, M. Dułak, L. Ferrighi, J. Gavnholt, C. Glinsvad, V. Haikola, H. A. Hansen, H. H. Kristoffersen, M. Kuisma, A. H. Larsen, L. Lehtovaara, M. Ljungberg, O. Lopez-Acevedo, P. G. Moses, J. Ojanen, T. Olsen, V. Petzold, N. A. Romero, J. StausholmMøller, M. Strange, G. A. Tritsaris, M. Vanin, M. Walter, ¨kkinen, G. K. H. Madsen, R. M. Nieminen, B. Hammer, H. Ha J. K. Nørskov, M. Puska, T. T. Rantala, J. Schiøtz, K. S. Thygesen and K. W. Jacobsen, J. Phys.: Condens. Matter, 2010, 22, 253202. 45 S. R. Bahn and K. W. Jacobsen, Comput. Sci. Eng., 2002, 4, 56–66. ¨chl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 46 P. E. Blo 50, 17953–17979. ¨chl, C. J. Fo ¨rst and J. Schimpl, Bull. Mater. Sci., 2003, 47 P. E. Blo 26, 33–41. ¨kkinen, L. Lehtovaara, M. Puska, J. Enkovaara, 48 M. Walter, H. Ha C. Rostgaard and J. J. Mortensen, J. Chem. Phys., 2008, 128, 244101. 49 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868. 50 M. D. Hanwell, D. E. Curtis, D. C. Lonie, T. Vandermeersch, E. Zurek and G. R. Hutchison, J. Cheminf., 2012, 4, 17. 51 A. K. Rappe, C. J. Casewit, K. S. Colwell, W. A. Goddard III and W. M. Skiff, J. Am. Chem. Soc., 1992, 114, 10024–10035. 52 S. D. Mo and W. Y. Ching, Phys. Rev. B: Condens. Matter Mater. Phys., 1995, 51, 13023–13032. 53 M. Anpo, T. Shima, S. Kodama and Y. Kubokawa, J. Phys. Chem., 1987, 91, 4305–4310. 54 S. Monticone, R. Tufeu, A. V. Kanaev, E. Scolan and C. Sanchez, Appl. Surf. Sci., 2000, 162, 565–570.
Phys. Chem. Chem. Phys.
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
PAPER
Cite this: DOI: 10.1039/c4cp02789b
View Journal
A DFT study of the effect of OH groups on the optical, electronic, and structural properties of TiO2 nanoparticles Olga Miroshnichenko,†a Sami Auvinen*b and Matti Alataloa The effects of on-surface OH groups on the structural and optical properties of small TiO2 particles have been studied in order to obtain knowledge about the optical behaviour of the TiO2 nanoparticles in solutions. The standard density functional theory was used to model the structural changes, and time-
Received 25th June 2014, Accepted 9th January 2015
dependent density functional theory was used to address the changes in the photoabsorption characteristics of an anatase-structured (TiO2)16 cluster. It was shown that the OH groups can alter both
DOI: 10.1039/c4cp02789b
the geometric and electronic structure of the clusters, resulting in changes in the optical properties. The
www.rsc.org/pccp
shown to be reduced by OH adsorption.
large blue shift, obtained in earlier calculations for TiO2 nanoparticles as compared with bulk TiO2, is
1 Introduction Titanium dioxide (TiO2) is one of the most investigated and most widely used metal oxides. Such popularity comes from the broad variety of applications of this wide band gap semiconductor in an enormous number of fields of industry and technology. These applications can be classified into energy and environmental applications. Titanium dioxide is used in solar cells,1–3 photocatalytic splitting of water under ultraviolet (UV) light,4–6 removal of organic pollutants, and in photo- and electrochromics.7–10 Its biological and chemical inertness and affordability explain the abundant use of TiO2 as a pigment11 in paints and food dyes.12 Its material properties depend strongly on the manufacturing method of the particles and on their geometrical and electronic structure.13–16 Relatively large particles with a size of approximately 200 nm are opaque and extremely white, which explains the usage of TiO2 as a pigment. When their size is decreased down to 10 nm, the particles become transparent, while still maintaining their ability to absorb UV-radiation, which makes them useful in sunscreens and UV-protectors.17,18 One of the main manufacturing methods of TiO2 nanoparticles is based on hydrolysis from various solutions or heating in water. Moreover, many applications and experiments with TiO2 take place in aqueous environments.19–21 Studies of water adsorption and therefore the adsorption of hydroxyl groups (OH) a
Department of Physics, University of Oulu, P.O. Box 3000, FI-90014, Oulu, Finland Faculty of Technology, Department of Mathematics and Physics, Lappeenranta University of Technology, P.O. Box 20, FI-53851, Lappeenranta, Finland. E-mail: [email protected] † Also at: Department of Theoretical Mechanics, St. Petersburg State Polytechnical University, Polytechnicheskaya 29, 195251, St. Petersburg, Russian Federation b
This journal is © the Owner Societies 2015
are thus highly important. A lot of experimental and theoretical studies on the adsorption of water22–29 and other molecules on titanium dioxide surfaces have been conducted. Bredow and Jug30 showed that the dissociative adsorption of water on rutile and anatase surfaces is more stable than molecular adsorption, and, moreover, molecularly adsorbed water is not active in photoreactions. Dissociatively adsorbed water plays an important role in the first steps of the photocatalytic degradation of organic water contaminants.31 In the early stages of precipitation, OH groups also take part in TiO2 structure formation and particle growth.32 Other studies have shown a big influence of adsorption on the optical properties of the material.33–36 For example, H adsorption causes a defect state 0.94 eV below the bottom of the conduction band.37 It was also shown that the doping or sensitizing of titanium dioxide extends its photoactivity to the visible light region.38,39 We have shown in our earlier work40,41 that the geometric structure of the clusters plays an important role in determining the optical properties. It is therefore necessary to use computational methods which address the geometric, electronic and optical properties simultaneously. Such an approach is provided by density functional theory (DFT), which, together with time-dependent DFT (TDDFT) can describe these properties with great accuracy. In the present work, we study the effect of the OH coverage on a (TiO2)16 nanocluster by calculating the geometric, electronic, and optical properties for the structure covered by one, three, six, and sixteen OH groups. This particular cluster was chosen since it is sufficiently small to allow a set of calculations to be performed in a reasonable time, yet large enough to be non-trivial. In our previous studies we have performed calculations comparing TiO2 nanoparticle structures
Phys. Chem. Chem. Phys.
View Article Online
Paper
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
which are as symmetrical as possible and more needle-like ones, and in all tested cases, more symmetrical structures were energetically more favorable.40 Based on these results, and results by Persson et al.,42 we selected a (TiO2)16 cluster for our test case. The shape, size, and structure of the selected cluster provides us with a reliable base for studies since it is large enough to have a clear anatase structure, it is stoichiometric, and the bonding environment is reasonably defect free with regard to the number of dangling surface bonds.42
2 Computational details All structural and electronic calculations in this study have been performed using the GPAW software package.43–45 GPAW is an efficient density functional theory based real space code using the projector-augmented wave (PAW) method.46,47 For the photoabsorption spectrum calculations we have used the TDDFT implementation48 in GPAW. All calculations were performed at zero Kelvin electronic temperature. The cluster structures were relaxed using a quasi-Newton minimizer so that all forces were smaller than 0.01 eV Å 1. The width of the Fermi-distribution was set to 25 meV in order to avoid the convergence problems related to some of the model structures. We used a grid spacing of 0.17 Å, and the computational cell was set up with non-periodic boundary conditions, with 7 Å of empty space surrounding the cluster. The cluster calculations were spin-paired using only the G-point. We used the conjugate gradient method for the eigensolver, and the Perdew, Burke, Ernzerhof (PBE)49 exchange correlation functional. All calculations were charge neutral. The density of states (DOS) plots for the clusters were obtained using basically the same computational settings as the structural optimization runs. In addition, the empty states were also required to be fully converged up to the ten highest bands with cluster calculations. The DOS were acquired with 2000 data points and a normalized Gaussian broadening of 0.1 eV. In order to further study the gap states found in the DOS results, we also performed atomic orbital projected DOS (PDOS) computations. The PDOS were acquired with the same computational settings as the DOS runs. When calculating the photoabsorption spectra we used the timepropagation TDDFT approach (TP-TDDFT). The spectra were calculated using a 16.0 attosecond time step with 1000 iterations, yielding a total simulation time of 16 femtoseconds. The grid-spacing was set to 0.3 Å, and the computational cell was set up with non-periodic boundary conditions, having 10 Å of empty space surrounding the cluster. The kick parameter for the initial distribution applied to the wave functions was 10 3. The spectra were calculated from the dipole moment files using a Gaussian broadening of 0.1 eV.
3 Results and discussion 3.1
Structural effects
In this work we address the effect of OH groups on the Ti16O32 structure, which was constructed so that all atoms should have
Phys. Chem. Chem. Phys.
PCCP
sufficient coordination. The relaxed structure is depicted in Fig. 1(a). We modelled the effect of one adsorbed OH group, along with three, six and 16 adsorbed OH groups on the cluster structure. The relaxed structures with adsorbed OH groups are presented in Fig. 1(b)–(e). Based on the results of Bredow and Jug30 we selected the on-surface titanium atoms as an adsorption sites for the OH-groups. The OH adsorption site in the case of one OH group was chosen using Avogadro molecular editor.50 The structures with hydroxyls adsorbed on all possible sites were test-relaxed with the built-in optimizer in Avogadro, using the Universal Force Field,51 which is appropriate for the full periodic table, including transition metals such as Ti. Five structures with minimal energies were relaxed in GPAW. The structure with the lowest energy became an input for electronic structure and optical property calculations in GPAW. Structures with 3 OH groups were tested only in Avogadro, due to the large amount of possible configurations and computational time limitations. With 6 OH groups the adsorption sites were chosen in order for the structure to be as symmetrical as possible. In the case of 16 OH groups, hydroxyls were attached to every Ti atom in the cluster. The structural data of the model particles has been collected in Tables 1–4. In the analysis of the bonds in the structures we have assumed that there is a Ti–O bond if the bonding distance is smaller than 2.2 Å, a Ti–Ti bond if the bonding distance is smaller than 2.94 Å, and an O–O bond if the bonding distance is smaller than 1.46 Å. Please note that in Tables 1–3 we have neglected the OH groups (bonds in OH groups and bonds between the cluster and OH groups) in order to see the structural changes in the underlying cluster structures; in Table 4 bonds between the cluster and OH groups are included in the analysis. The number of bonds in the structures after the relaxation are presented in Table 1. As we can see, the general trend is that the adsorption of OH groups tends to break or stretch Ti–Ti and Ti–O bonds in the structures, which is visible in Table 1 as a lower number of bonds in the relaxed structures with adsorbed hydroxyls relative to the number of bonds in the relaxed bare cluster. On the other hand, the average bond lengths in the remaining Ti–Ti and Ti–O bonds do not change dramatically, as we can see in Table 2. In structures with 3, 6 and 16 OH groups the total average length of all bonds seems to change more due to the appearance of one O–O bond in these structures. The relative percentage changes in three dimensions and average dimensions of the structures have been collected in Table 3. By looking at the data in Tables 1–3, we can conclude that the adsorbed OH groups reshape the TiO2 clusters by breaking or extending Ti–Ti and Ti–O bonds, so that the average dimension of the particles is increased. Regarding the coordination environment of the nanoparticles, the coordination data of the studied cluster structures is collected in Table 4. In the coordination analysis we have neglected the oxygen and hydrogen atoms in the on-surface OH groups, but the bonds between the surface titanium atoms and the oxygen atoms in the on-surface OH groups have been
This journal is © the Owner Societies 2015
View Article Online
Paper
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
PCCP
Fig. 1 The relaxed cluster structures: bare Ti16O32 (a), Ti16O32 with one OH group (b), Ti16O32 with three OH groups (c), Ti16O32 with six OH groups (d), and Ti16O32 with 16 OH groups (e). Oxygen atoms are marked with red color, titanium atoms with gray color, and hydrogen atoms are blue. The atoms corresponding to the observed defect states are numbered and emphasized with black rectangles. The red, green, and blue coordinate axes represent the x, y, and z axes correspondingly.
calculated for the coordination of the titanium atoms. This has been done in order to be able to see the effect of the adsorbed OH groups on the coordination of the underlying cluster structures. As a reference we have used the coordination of titanium and oxygen in bulk TiO2, where the coordination of titanium is
This journal is © the Owner Societies 2015
6-fold and the coordination of oxygen is 3-fold.52 Normally the coordination number of oxygen does not exceed 3, and if the coordination number is higher than three the bonding is ionic, whereas if the coordination number is under three the bonding is regarded as covalent.38
Phys. Chem. Chem. Phys.
View Article Online
Paper
PCCP
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
Table 1 Number of bonds in the relaxed structures. The OH groups are not taken into account
Structure
nTi–O
nTi–Ti
nO–O
nAll
Ti16O32 Ti16O32 Ti16O32 Ti16O32 Ti16O32
68 66 65 62 58
12 11 9 2 1
0 0 1 1 1
80 77 75 65 60
bare + OH + 3OH + 6OH + 16OH
Table 2 Average bond lengths in the relaxed structures. The OH groups are not taken into account
Structure
dTi–O/Å
dTi–Ti/Å
dO–O/Å
All/Å
Ti16O32 Ti16O32 Ti16O32 Ti16O32 Ti16O32
1.872 1.867 1.878 1.868 1.882
2.789 2.832 2.822 2.810 2.848
— — 1.443 1.452 1.334
2.009 2.005 1.985 1.891 1.889
bare + OH + 3OH + 6OH + 16OH
Table 3 Percentage changes in the dimensions of the relaxed clusters in three directions (x, y and z) relative to the relaxed bare TiO2 structure. Positive and negative values correspondingly stand for increases and decreases in the dimensions. The OH groups are not taken into account
Structure Ti16O32 Ti16O32 Ti16O32 Ti16O32
+ + + +
x OH 3OH 6OH 16OH
y
7.4 13.07 30.98 0.15
4.5 0.74 10.44 11.88
z
Avg.
14.5 6.37 12.26 31.83
4.8 5.26 7.36 20.07
Table 4 Coordination of Ti and O atoms in relaxed clusters. Atoms in the OH groups are not taken into account
Structure
3f-Ti
4f-Ti
5f-Ti
6f-Ti
7f-Ti
1f-O
2f-O
3f-O
Ti16O32 Ti16O32 Ti16O32 Ti16O32 Ti16O32
— 1 — — —
— 1 1 9 7
6 5 9 6 8
8 7 5 1 1
2 2 1 — —
— 1 — 2 5
28 28 29 28 26
4 3 3 2 1
bare + OH + 3OH + 6OH + 16OH
As we can see in the Table 4 the number of undercoordinated oxygen atoms varies within 3 bonds, and the number of under-coordinated titanium atoms within 9 bonds. The number of over-coordinated titanium atoms varies within 2 bonds. The total number of all under-coordinated atoms varies from 34 to 46, and the clusters with highest total number of under-coordinated atoms (structures with 6 and 16 adsorbed OH groups) are the ones with the highest photoabsorptions above 12.5 eV. The cluster with 46 under-coordinated atoms (cluster with 16 adsorbed hydroxyls) shows the highest photoabsorption also between 4 and 8.5 eV (Fig. 3). 3.2
Electronic properties
The DOS plots for the Ti16O32 structures with one, three, six, and sixteen adsorbed OH groups are presented in Fig. 2 along with the DOS of the bare structure for reference, and the band gap results are collected in Table 5. The DOS of bare Ti16O32 shows a clear band structure with a well defined valence band
Phys. Chem. Chem. Phys.
Fig. 2 DOS of bare Ti16O32 (a), Ti16O32 with one OH group (b), Ti16O32 with three OH groups (c), Ti16O32 with six OH groups (d), and Ti16O32 with 16 OH groups (e). HOMO and LUMO levels are plotted with vertical dashed lines. In (b), (c), and (e) HOMO and LUMO levels almost coincide.
(VB) and conduction band (CB),‡ which corresponds well to the results by Persson et al.,42 who found that stoichiometric, adequately coordinated TiO2 clusters will have well defined, defect-free band gaps. Adding one OH group introduces empty states just above the edge of the VB, resulting in an almost negligible difference between the HOMO and LUMO levels (DHOMO–LUMO). This is interesting since the fundamental band gap (separation between VB and CB) still remains as large as 2.947 eV, which is actually almost 0.114 eV larger than the DHOMO–LUMO of the bare cluster. The PDOS results reveal that the empty state above the valence band is an Op state of the oxygen atom 1 in Fig. 1(b). The adsorbed OH group alters the cluster structure during the relaxation so that oxygen atom 1 has been separated from one titanium atom. This results in a terminal Ti–O bond, which is energetically unfavourable, and the breaking of the bulk symmetry causes the empty Op state to appear at the edge of the valence band. ‡ Please note, that here we use the notation ‘‘band’’, but in fact the states are more localized than continuous in the case of very small clusters.
This journal is © the Owner Societies 2015
View Article Online
PCCP
Paper
Table 5 Electronic structure information for the Ti16O32 clusters (all energies are in eV). All structures are relaxed
Structure Ti16O32 Ti16O32 Ti16O32 Ti16O32 Ti16O32
+ + + +
OH 3OH 6OH 16OH
HOMO
LUMO
0.626 0.033 0.121 0.884 0.014
2.207 0.024 0.001 1.460 0.006
DHOMO–LUMO
Eg
2.833 0.009 0.120 2.344 0.020
2.833 2.947a 2.875a 2.344 2.360a
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
a The gap (Eg) was calculated as the difference between the edges of the valence and conduction bands.
The adsorption of three OH groups causes the appearance of empty states just below the Fermi level; these states are Op states of the oxygen atoms 2 and 3 in Fig. 1(c), caused probably by changes in the coordination of the surrounding titanium atoms during the relaxation. In the case of six adsorbed OH groups there are clear empty gap states around 1.5 eV, which lowers DHOMO–LUMO. PDOS results show that these empty states are d states of titanium atom 4 in Fig. 1(d). These states are due to the lower coordination of titanium atom 4 after the relaxation, and due to the exceptional triple O coordination environment (oxygen atoms 5, 6, and 7 in Fig. 1(d)) next to it. In the case of 16 adsorbed OH groups, the almost negligible DHOMO–LUMO is again due to the empty Op states of the oxygen atoms 8 and 9 in Fig. 1(e). Here the empty states are located at the edge of the valence band due to the O–O bond of oxygen atoms 8 and 9. Adding more OH groups on the cluster surface clearly shifts the intensity of the DOS so that the VB states become more localized at the lower edge of the VB (from 8.5 to 4 eV), and the CB states become more localized at the upper edge of the CB (from 6 to 9.5 eV). The appearance of new states at the lower part of VB is due to the contribution of the oxygen atoms in the OH groups. The increase in the intensity of the DOS in the upper edge of the CB is caused by the increase in the number of atoms in the system; in this energy region all atoms contribute to the DOS without the explicit prevalence of particular atoms. In the case of one, three, and sixteen adsorbed OH groups the DHOMO–LUMO value of the system is almost negligible, but in the case of six adsorbed OH groups, the DHOMO–LUMO value is 2.34 eV. The symmetry of the structures can affect the electronic structure so that values of DHOMO–LUMO decrease. Structures with one and three adsorbed hydroxyls are not symmetrical, which results in an almost zero DHOMO–LUMO value. The structure with 16 OH groups undergoes strong rearrangment during the structure relaxation, inducing defects in the structure. These defects may be responsible for the lowering of the DHOMO–LUMO value. In the case of the Ti16O32 structure, we can use the computed and estimated band gaps, and the average is 2.67 eV. When this value is compared to our earlier computational result for the band gap of bulk anatase (2.12 eV),40 we see that there is an average gap broadening of 0.55 eV, which corresponds well with previously reported values of 0.1–0.6 eV.20,53,54 3.3
Fig. 3 Total averaged photoabsorption spectra for the bare Ti16O32 structure, and the Ti16O32 structure with one OH group, with 3 OH groups, with 6 OH groups, and with 16 OH groups.
clearly visible that the trend observed in the DOS results shows up also in the photoabsorption characteristics of the clusters. The on-surface OH groups enhance the absorption at lower energies (7–9 eV) and at higher energies (12.5–30 eV). One adsorbed OH group does not significantly alter the photoabsorption spectrum, although small enhancements at the above mentioned energies can be observed. Three and six adsorbed OH groups clearly alter the photoabsorption characteristics, and in the case of 16 adsorbed OH groups the change is already significant. In the inset of Fig. 3 the magnified spectra curves in the low energy region are presented. It can be seen that for structures with small differences between HOMO and LUMO, photoabsorption starts already at very low energy. However, since the density of empty states near the VB edge is low, the effect of the low energy transitions does not significantly alter the absorption threshold at larger scale. The total averaged (averaged over all coordinate axes) refractive index functions (RIFs) for the same structures are presented in Fig. 4. The derivation of RIFs from photoabsorption spectra is presented in ref. 41. What should be noted here is
Optical properties
The total averaged (averaged over all coordinate axes) photoabsorption spectra for all structures are presented in Fig. 3. It is
This journal is © the Owner Societies 2015
Fig. 4 Total averaged refractive index functions for the bare Ti16O32 structure, and the Ti16O32 structure with one OH group, with three OH groups, with six OH groups, and with 16 OH groups.
Phys. Chem. Chem. Phys.
View Article Online
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
Paper
that when calculating the RIF from the photoabsorption spectrum we suppose that the particle is purely anatase structured TiO2, and neglect the effect of OH groups on the density and valence electron density of the particle. However, this only affects the amplitude and level of the RIF, and the peaks and shape of the RIF remain unaffected. The effect of the OH groups on the dimension of the particles is included in the calculation of RIFs. The trend observed in the photoabsorption characteristics of the selected particles is also visible in the RIFs. The on-surface OH groups change the RIF at longer wavelengths (130–350 nm) and at shorter wavelengths (50–100 nm). With one adsorbed OH group small enhancements at the above-mentioned wavelengths can be observed, and with 16 adsorbed OH groups the changes are clearly visible. Previously we have reported significant blue shifts when the computational RIFs of the TiO2 nanoparticles were compared to the experimental bulk values.41 The present results for the photoabsorptions and RIFs of the Ti16O32(OH)n structures will complement our previous results, and show that the blue shifts in the case of the TiO2 clusters can be smaller when the clusters are covered with adsorbants, or are in aqueous solutions. Moreover, the results clearly reveal the fact that when considering the computationally modelled absorption properties of the TiO2 nanoparticles, the effects of the surrounding media will have to be taken into consideration. For example for the refractive index, the usual procedure to estimate the effect of the surrounding media for bare TiO2 nanoparticles ignores the effects of particle–adsorbate interaction.
4 Conclusions We have studied the effects of adsorbed OH groups on the atomic and electronic structures, photoabsorption spectra, and RIFs of Ti16O32 nanoparticles. The results show that adsorbed OH groups tend to break or stretch some near-surface Ti–Ti and Ti–O bonds in the underlying structures, while on the other hand the average bonding lengths of the remaining bonds do not change dramatically. The adsorbed OH groups cause an enhancement of the intensity of the DOS of the nanoparticle at the lower part of the valence band around 4 eV, whereas the intensity of the DOS of the conduction band is enhanced at the upper part of the band around 7 eV. The same trend is observed in the photoabsorption spectra of the Ti16O32 nanoparticles, where the absorption is increased at lower (7–9 eV) and higher energies (12–30 eV). The trend observed in the DOS and photoabsorption characteristics is also visible in the RIFs of the particles. The results indicate that the effects of the surrounding media have to be taken into consideration when considering the computationally modelled absorption properties of small TiO2 nanoparticles.
PCCP
Technology, and Sachtleben Pigments Oy,§ Pori, Finland for funding this research. We would also like to acknowledge Juho¨ki, Pertti Jalava, PhD from Prof. Math Oy, Ralf-Johan Lamminma PhD, and Minna Kuusisto, MSc, from Sachtleben Pigments Oy,§ and Dr Erik Vartiainen from Lappeenranta University of Technology for their comments. The computational resources were provided by CSC-Scientific Computing Ltd, Espoo, Finland.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
¨tzel, Prog. Photovoltaics, 2000, 8, 171–185. M. Gra ¨tzel, J. Photochem. Photobiol., C, 2003, 4, 145–153. M. Gra C. G. Granqvist, Adv. Mater., 2003, 15, 1789–1803. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38. A. Fujishima, T. N. Rao and D. A. Tryk, J. Photochem. Photobiol., C, 2000, 1, 1–21. D. A. Tryk, A. Fujishima and K. Honda, Electrochim. Acta, 2000, 45, 2363–2376. ¨tzel, Nature, 2001, 414, 338–344. M. Gra ¨tzel, Chem. Rev., 1995, 95, 49–68. A. Hagfeldt and M. Gra A. L. Linsebigler, G. Lu and J. J. T. Yates, Chem. Rev., 1995, 95, 735–758. A. Mills and S. L. Hunte, J. Photochem. Photobiol., A, 1997, 108, 1–35. G. Pfaff and P. Reynders, Chem. Rev., 1999, 99, 1963–1981. J. H. Braun, A. Baidins and R. E. Marganski, Prog. Org. Coat., 1992, 20, 105–138. A. P. Alivisatos, J. Phys. Chem., 1996, 100, 13226–13239. A. P. Alivisatos, Science, 1996, 271, 933–937. C. Burda, X. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev., 2005, 105, 1025–1102. Y. Hwu, Y. D. Yao, N. F. Cheng, C. Y. Tung and H. M. Lin, Nanostruct. Mater., 1997, 9, 355–358. A. Salvador, M. C. Pascual-Marti, J. R. Adell, A. Requeni and J. G. March, J. Pharm. Biomed. Anal., 2000, 22, 301–306. R. Zallen and M. P. Moret, Solid State Commun., 2006, 137, 154–157. A. Henglein, Top. Curr. Chem., 1988, 143, 113–180. C. Kormann, D. W. Bahnemann and M. Hoffmann, J. Phys. Chem., 1988, 92, 5196–5201. ¨tzel, J. Am. Chem. D. Duonghong, E. Borgarello and M. Gra Soc., 1981, 103, 4685–4690. R. L. Kurtz, R. Stockbauer, T. E. Madey, E. Roman and J. L. de Segovia, Surf. Sci., 1989, 218, 178–200. M. J. Jaycock and J. C. R. Waldsax, J. Chem. Soc., Faraday Trans. 1, 1974, 70, 1501–1517. M. Tsukada, H. Adachi and C. Satoko, Prog. Surf. Sci., 1983, 14, 113–174. J. Goniakowski, S. Bouette-Russo and C. Noguera, Surf. Sci., 1993, 284, 315. A. Fahmi and C. Minot, Surf. Sci., 1994, 304, 343. B. Boddenberg and W. Horstmann, Ber. Bunsenges. Phys. Chem., 1988, 92, 519.
Acknowledgements
26 27
We want to acknowledge the Finnish Academy of Science and Letters, the Research Foundation of Lappeenranta University of
§ Huntsman Pigments from 1.10.2014.
Phys. Chem. Chem. Phys.
This journal is © the Owner Societies 2015
View Article Online
Published on 12 January 2015. Downloaded by Selcuk University on 26/01/2015 15:47:20.
PCCP
28 S. Bourgeois, F. Jomard and M. Perdereau, Surf. Sci., 1992, 279, 349–354. 29 Y. Suda and T. Morimoto, Langmuir, 1987, 3, 786–788. 30 T. Bredow and K. Jug, Surf. Sci., 1995, 327, 398–408. 31 C. S. Turchi and D. F. Ollis, J. Catal., 1990, 122, 178–192. 32 M. Gopal, W. J. M. Chan and L. C. D. Longhe, J. Mater. Sci., 1997, 32, 6001–6008. 33 K. L. Miller, C. B. Musgrave, J. L. Falconer and J. W. Medlin, J. Phys. Chem. C, 2011, 115, 2738–2749. 34 A. Iacomino, G. Cantele, D. Ninno, I. Marri and S. Ossicini, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 075405. 35 W. Zeng, T. Liu, Z. Wang, S. Tsukimoto, M. Saito and Y. Ikuhara, Mater. Trans., 2010, 51, 171–175. 36 C. O’Rourke and D. Bowler, J. Phys. Chem. C, 2010, 114, 20240–20248. 37 C. D. Valentin and G. Pacchioni, J. Phys. Chem. C, 2009, 113, 20543–20552. 38 Z. W. Qu and G. J. Kroes, J. Phys. Chem. B, 2006, 110, 8998–9007. 39 S. A. Shevlin and S. M. Woodley, J. Phys. Chem. C, 2010, 114, 17333–17343. 40 S. Auvinen, M. Alatalo, H. Haario, J.-P. Jalava and R.-J. ¨ki, J. Phys. Chem. C, 2011, 115, 8484–8493. Lamminma 41 S. Auvinen, M. Alatalo, H. Haario, J.-P. J. E. Vartiainen and ¨ki, J. Phys. Chem. C, 2013, 117, 3503–3512. R.-J. Lamminma 42 P. Persson, J. Gebhardt and S. Lunell, J. Phys. Chem. B, 2003, 107, 3336–3339. 43 J. J. Mortensen, L. B. Hansen and K. W. Jacobsen, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 71, 035109.
This journal is © the Owner Societies 2015
Paper
44 J. Enkovaara, C. Rostgaard, J. J. Mortensen, J. Chen, M. Dułak, L. Ferrighi, J. Gavnholt, C. Glinsvad, V. Haikola, H. A. Hansen, H. H. Kristoffersen, M. Kuisma, A. H. Larsen, L. Lehtovaara, M. Ljungberg, O. Lopez-Acevedo, P. G. Moses, J. Ojanen, T. Olsen, V. Petzold, N. A. Romero, J. StausholmMøller, M. Strange, G. A. Tritsaris, M. Vanin, M. Walter, ¨kkinen, G. K. H. Madsen, R. M. Nieminen, B. Hammer, H. Ha J. K. Nørskov, M. Puska, T. T. Rantala, J. Schiøtz, K. S. Thygesen and K. W. Jacobsen, J. Phys.: Condens. Matter, 2010, 22, 253202. 45 S. R. Bahn and K. W. Jacobsen, Comput. Sci. Eng., 2002, 4, 56–66. ¨chl, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 46 P. E. Blo 50, 17953–17979. ¨chl, C. J. Fo ¨rst and J. Schimpl, Bull. Mater. Sci., 2003, 47 P. E. Blo 26, 33–41. ¨kkinen, L. Lehtovaara, M. Puska, J. Enkovaara, 48 M. Walter, H. Ha C. Rostgaard and J. J. Mortensen, J. Chem. Phys., 2008, 128, 244101. 49 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865–3868. 50 M. D. Hanwell, D. E. Curtis, D. C. Lonie, T. Vandermeersch, E. Zurek and G. R. Hutchison, J. Cheminf., 2012, 4, 17. 51 A. K. Rappe, C. J. Casewit, K. S. Colwell, W. A. Goddard III and W. M. Skiff, J. Am. Chem. Soc., 1992, 114, 10024–10035. 52 S. D. Mo and W. Y. Ching, Phys. Rev. B: Condens. Matter Mater. Phys., 1995, 51, 13023–13032. 53 M. Anpo, T. Shima, S. Kodama and Y. Kubokawa, J. Phys. Chem., 1987, 91, 4305–4310. 54 S. Monticone, R. Tufeu, A. V. Kanaev, E. Scolan and C. Sanchez, Appl. Surf. Sci., 2000, 162, 565–570.
Phys. Chem. Chem. Phys.
