Chapter 5: Intrinsic Defect Induced Room Temperature
6.8. Discussion
There are several reports on the siz e effect of lattice parameter variation in metal oxides nanocrystals.1-4 , 6, 7 Recently, it was reported38 that the oxygen vacancy formation by itself, without a cation radius change in CeO2 results in a contraction of lattice parameters on the basis of molecular dynamic simulation. It was explained that upon the formation of oxygen vacancy, the cation relaxed away from the vacancy (by approximately 0.10 Å ), however the anion got closer to it (by approximately 0.16 Å ) and the magnitude of the significant distortion resulted in a net local contraction of the lattice around the vacancy. However, the combined effects of oxygen vacancy and associated cation radii change results in lattice expansion of CeO2.38 In particular, the lattice expansion and contraction of rutile and anatase TiO2 was reported by various groups.1-3, 6 L i et al.2 reported that particle siz e reduction induces lattice contraction due to surface hydration effect in pure anatase TiO2 nanocrystals.
On the other hand, S wamy et al.3 observed lattice expansion at reduced crystallite siz e in anatase TiO2 and explained the lattice expansion in terms of increased Ti vacancy and lattice strain with decreasing crystallite siz e. M oreover, lattice expansion with decreasing crystallite siz e is reported in rutile TiO2.1, 6 A surface defect dipole model was proposed by L i et al.1 to explain the lattice expansion of rutile TiO2 nanocrystals with the reduction in siz e in terms of the strong interaction among the surface dipoles that produced an increased negative pressure. This is in contrast to the ionicity increase induced lattice expansion in BaTiO3
proposed by Tsunek awa et al.5 The siz e dependent lattice expansion of rutile TiO2 was explained in terms of Ti vacancy abundance by K uz netsov et al.6 Using ab initio calculation,
Iddir et al.14 predicted that for , the significant displacement extended along the [110] and [001] directions of the eq uatorial plane encompassing the vacancy. The three Ti nearest neighbor have the largest outward displacement of 0.31 Å and the nearest O neighbor of have the largest inward displacement of 0.14 Å . It was also reported14 that for , the major displacement occurred in the plane perpendicular to [001], the largest being observed in the plane passing through . The two nearest neighbor Ti are pushed away from their ideal position by 0.3 Å and the closest four O atoms relax toward the by 0.21 Å . The lattice atoms relaxed around the defects to accommodate the induced stress due to the diffusion of through the interstitial region. Based on the above information, we discuss the atomistic origin of lattice expansion and contraction of rutile TiO2 in terms of intrinsic defects rather than siz e induced lattice variations, as the siz e of our nanostructure is relatively large compared to those reported in the literature.1, 6 The intrinsic defects include, and in our rutile TiO2 NS .
F irst, we discuss the possible mechanisms of lattice expansion in our rutile TiO2 NS . F rom Fig. 6.2(b ) and T a b le 6.1, it is seen that the lattice is expanded as the growth temperature is increased. G enerally, in nanoscale regime, the lattice expansion in metal oxide nanocrystals is believed to result from complex interplay of defect chemistry and physics.
F or example, (i) increase in ionic radius upon the reduction of cation valence state might cause the lattice expansion, and (ii) the electrostatic repulsion between positively charged oxygen vacancy and metal cations surrounding it may also be responsible for the lattice expansion. E S R and X P S measurement on our samples do not show any signature of , which eliminates the possibility of lattice expansion due to reduction of cationic radii.
However, E S R signal shows the evidence of (singly ioniz ed oxygen vacancy) center in our samples. In our previous report and C hap te r 5, the pathways for the formation of and trap center were explained.30 Note that we observed related NIR P L and and neutral related visible P L emission in our samples. The Ti-Ti and Ti-O bonds are relaxed due to the missing O atom. The nearest-neighbor Ti atoms move outward from the vacancy to strengthen the bonding with rest of the neighboring O lattice. W hile the next-nearest- neighboring O atoms move inward due to the absence of electrostatic repulsion by the missing O atom. It was reported39 , 4 0 that the local distortion due to the Ti atom displacement
depends upon the different charge state of oxygen vacancy, i.e. neutral, singly charged (+1) and doubly charged (+2) oxygen vacancy. The positive charges cause greater repulsion for Ti-Ti distance compared to neutral oxygen vacancy.39 , 4 0 Note that we observed singly charged oxygen vacancy () in E S R measurement and both neutral and singly charged in P L measurement. S o, we expect the lattice expansion in our samples to be caused by the oxygen vacancies creating and neutral defects. The electrostatic repulsion between and helps in migration of the unsaturated from the bulk towards the surface to interact with the atmospheric O2 and form during the course of air annealing and occupying an interstitial site similar to the lattice Ti site in octahedral TiO2 crystal system. It was reported from both experimental and theoretical analyses that diffusion is faster in rutile TiO2 than the diffusion when annealing in O2 atmosphere.4 1-4 3 The lattice Ti is strongly bonded with the lattice O atoms. W hen the is placed in the interstitial site, the Ti atoms are relaxed outward due to the electrostatic repulsion between positively charged Ti ions. However, the lattice O atoms are slightly distorted towards the as they are strongly bonded to lattice Ti atoms. As a result, the net relaxation favors the expansion of lattice volume which is due to the presence of defects. It is noticed that the increases with the increase in growth temperature from the P L spectra suggesting more lattice expansion.
S o, the lattice expansion is observed in our samples in the order C18> B18> A18. The sample A18 has an additional P L peak at 1.96 eV compared to B18 and C18 which is related to and will be discussed in the following paragraph. Our experimental results regarding the lattice expansion due to and are consistent with the theoretical prediction of Iddir et al.14
S econdly, we discuss the possible mechanism behind the lattice contraction in rutile TiO2 NS . Our P L study on TiO2 NS showed that longer the reaction duration, higher the and defects for the same growth temperature.29 This is also reflected in the present case, as related P L emission is enhanced for the longer reaction duration samples. S ince the oxygen vacancies are more in these samples, it is expected that the atmospheric O2 may inject rapidly into the TiO2 to occupy the oxygen vacancy positions during the course of air annealing at 500 ° C, as a result the oxygen vacancy concentration decreased considerably after annealing, which is reflected in our P L spectra (reduction of broad visible P L in samples
A18 and A4 8 compared to A12). It was recently reported that adsorbed isotopic oxygen is injected into the bulk rutile TiO2 as which underwent drift towards the surface due to space-charge layers and retarded the diffusion of interstitials, promoting the exchange with lattice oxygen.4 4 , 4 5 P resumably most of the near the surface are annihilated due to diffusion of O2 and shut down the path for further diffusing and creating separate ions which facilitates creation of surface electronic bands. The P L emission at 1.96 / 1.99 eV observed in A18/A4 8, may be attributed to the surface electronic bands created by ions.
The electrostatic attraction between the and is dominant over the lattice expansion due to and neutral which results in the lattice contraction for the samples A18 and A4 8 compared to A12.
Fig. 6.9 . S chematic diagram illustrating lattice expansion and lattice contraction in TiO2 lattice due to intrinsic defects. G rey (large) balls are Ti atoms and red (small) balls are oxygen atoms. The light pink (small) ball indicates the oxygen vacancy. The green (large) and black (small) ball indicate and , repectively. The arrow shows the direction of relaxation of atoms due to , and defects.
Another interesting point to be noted is that the red P L at 1.96 eV is absent for the sample B18 and C18 grown at higher growth temperature. It was shown in C hap te r 4 that for growth at higher temperature, P L due to was much stronger than the P L due to .29 D ue to lower concentration of , the atmospheric O2 interacts with and oxidiz ed to instead of injecting into the bulk and this may be a possible reason for the absence of related P L peak at 1.96 eV (1.99 eV) in B18 (C18). The lattice expansion of A18s compared to A18 can be explained on basis of higher concentration of and neutral and absence of , which results in net electrostatic repulsion between and Ti cations. The lattice contraction due to surface hydroxyl group in pure anatase TiO22 is inconsistent with the lattice expansion in our rutile TiO2 samples A12 and A18s, which are associated with adsorbed surface OH group, as confirmed from X P S studies. Therefore, we strongly believe that the intrinsic defects, such as , and are primarily responsible for lattice parameter variation in our rutile TiO2 NS . A schematic diagram illustrating the lattice expansion due to the presence of oxygen vacancy and Ti interstitial defects, whereas lattice contraction due to the electrostatic attraction between the and defects is shown in Fig. 6.9. In case of mediated expansion, due to the missing O atom, the Ti-Ti and Ti-O bonds are relaxed. The nearest-neighbor Ti atoms move outward from the vacancy to strengthen the bonding with rest of the neighboring O lattice. Though the next-nearest- neighboring O atoms may move inward to fill the site, the net outward movement of Ti atoms is higher than the net inward movement of O atoms. This results in lattice expansion.
L attice expansion due to Ti interstitial is more natural due to larger siz e of the Ti atoms and electrostatic repulsion between Ti4 + ions. On the other hand, lattice contraction is caused by the electrostatic attraction between and , as shown by the green arrow. Note that the req uirement of a small siz e below a critical siz e is not imperative in this mechanism. This calls for a revisit of the earlier reported results on the siz e dependent lattice expansion in TiO2 NS .