Probing Raman Enhancement in a DopamineâTi2O4 Hybrid Using

Article pubs.acs.org/JPCA

Probing Raman Enhancement in a Dopamine−Ti2O4 Hybrid Using Stretched Molecular Geometries Inés Urdaneta,*,†,‡,§ Julien Pilmé,†,‡ Arne Keller,§ Osman Atabek,§ Pilarisetty Tarakeshwar,∥ Vladimiro Mujica,∥,⊥,# and Mónica Calatayud†,‡,¶ †

Laboratoire de Chimie Théorique, UPMC Univ Paris 06, UMR 7616, F-75005 Paris, France Laboratoire de Chimie Théorique, CNRS, UMR 7616, F-75005 Paris, France § Institut des Sciences Moléculaires, CNRS and UMR8214, Bât 350, Université Paris Sud, 91405 Orsay, France ∥ Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604, United States ⊥ Department of Chemistry, Northwestern University, Evanston, Illinois 60208 United States # Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

ABSTRACT: Hybrids consisting of a metal oxide nanoparticle and a molecule show strong enhancement of Raman modes due to an interfacial charge transfer process that induces the formation of midgap states, thereby reducing the effective gap compared to that of the nanoparticle and creating the posibility of an electronic resonance at energies substantially lower than the nanoparticles’s band gap. We have developed a simple methodology to mimic the presence of the nanoparticle through a deformation of the bond involved in the chemical binding between the two entities forming the hybrid. The results provide a convincing interpretative frame to the enhancements observed in Raman spectra when all atoms are included. In addition, these enhancements can be correlated to a crossing of excited molecular orbitals that take part in the virtual excitation associated with the Raman process. We illustrate our method for the dopamine−Ti2O4 hybrid using the most acidic molecular O−H bond as the control parameter for the deformation.

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INTRODUCTION Surface enhanced Raman scattering (SERS) is a surface sensitive technique that results in the enhancement of Raman scattering by molecules adsorbed on semiconducting or rough metal surfaces. The SERS effect on metallic surfaces is mainly due to local intense field induced by localized plasmon resonance.1−4 In contrast, Raman enhancement in hybrids consisting of molecules adsorbed on metal oxide nanoparticles is mainly induced by charge transfer processes.5,6 In terms of applications, this type of system has awakened considerable interest in photovoltaics and as biosensors, because they are biocompatible.7−17 Organic molecules on TiO2 nanoparticles show an important increase of the Raman signal with respect to the isolated molecule.18,19,20,21,22 Noticeably, some enhanced modes involve atoms in the nanoparticles, a situation that differs from the metal−molecule cases where only molecular modes are enhanced. Recent experiments23,24 have shown that the hybrid system dopamine−TiO2 manifests a SERS effect. Theoretical studies25 with small TiO2 clusters have concluded that the role of the dopamine’s attachment to the surface is 2-fold: (i) it gives rise to new occupied states (coming mainly from the molecule) © 2014 American Chemical Society

inside the cluster’s original band gap, thereby reducing the effective band gap and opening an electronic resonance pathway for the Raman process, and (ii) it gives rise to a large charge polarization associated with the different localization patterns of the HOMO and LUMO orbitals of the complex (the HOMO is localized mainly in the molecule and the LUMO is mostly placed in the nanocluster). There is conclusive evidence that this charge transfer (CT) mechanism from the molecule to the nanoparticle is responsible for the increase of the polarizability gradient and therefore the Raman signal.25 It has been shown that dopamine’s hydrogen atoms iniatially migrate to the nanoparticle, a fact that has been supported by theoretical calculations on TiO2 clusters26 and very recently on TiO2 surfaces.29 Experimental27 and theoretical28 evidence is found for surfaces, regarding the dissociation of the dopamine OH groups. In ref 27 the orientation of dopamine molecules on the TiO2 surface is determined, and an attempt to elucidate the Received: November 1, 2013 Revised: December 12, 2013 Published: January 23, 2014 1196

dx.doi.org/10.1021/jp410781y | J. Phys. Chem. A 2014, 118, 1196−1202

The Journal of Physical Chemistry A

Article

Figure 1. Raman activity as a function of Raman frequency shift for the dopamine molecule (red solid line) and hybrid (blue dashed line). Inset: left column dopamine molecule, right column hybrid. First row (a, A): structure of the two systems. Second row (b, B): HOMO. Third row (c, C): LUMO. Color code: red for oxygen (O), yellow for hydrogen (H), turquoise for carbon (C), blue for nitrogen (N), and gray for titanium (Ti). The encircled region in (a) is the bond to be streched.

a critical value of the geometry parameters that is also accompanied with the crossing of excited orbital energies. This latter element, i.e., the orbital crossing, is the fundamental ingredient in the change in polarizability associated with the Raman activity enhancement because these excited orbitals play a critical role in the description of the resonant feature of the Raman response. Finally, we mention that there is increasing evidence in extended systems related to the hybrids we are considering, such as nanocrystals, in which the role of the adsorbate might not be to create midgap states but rather to produce shallow traps for exciton capture. The intriguing possibility of using Raman to probe these electronic states is clearly open and would contribute to the emergence of a better understanding of the way ligands modify the electronic states of nanosystems.31,32,33,34

charge transfer process is presented. It is deduced that both oxygen atoms in the dopamine molecule that are chemisorbed to the surface are in the same environment, which implies that bonding to the surface is through both oxygen atoms following deprotonation. Theoretical calculations sustain the experimental evidence, showing that the process obeys an acid−base adsorption mechanism28,29 where the surface oxygen sites would be more basic than the molecule ones, inducing the migration of the H groups to bind to the former. The adsorption process involves substantial changes in the molecular bonds and angles of the adsorbed molecule compared to those of the isolated species. This is particularly true for the geometrical parameters characterizing the geometry of bonds and atoms directly involved in the adsorption process. In turn, these deformations may have effects on the activity of specific Raman lines due to the modification of charge distribution induced by these distortions. Additionally, it has been reported for the dopamine molecule that one of the molecular O−H bonds happens to be more acidic than the other.30 Taking these facts into account, we show in this study that there is a strong qualitative correlation between the Raman spectra calculated for certain distorted molecular geometries and that of the oxide cluster−molecule hybrid. In particular, deformed geometries obtained by stretching the most acidic O−H bond in the isolated molecule render remarkably similar spectra to the ones calculated for the whole hybrid. This type of correlation can be rationalized by invoking the ideas of reaction field theory and considering the O−H distance as a generalized reaction coordinate. In this case one obtains an expression for the polarizability as a function of the bond distance parameter. We have carried out a systematic study of the Raman spectrum, orbital polarization, and HOMO−LUMO gap as a function of the O−H bound length d and found that, in all relevant cases, there is a discontinuity of Raman modes decomposition in terms of normal coordinates that occcurs at

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RAMAN SPECTRA OF THE ISOLATED MOLECULE AND THE HYBRID SYSTEM All DFT optimization and Raman calculations are performed using Gaussian09, with the B3LYP functional and a 6-31g(d,p) basis set. As a first step, we present the relevant results of the geometry and energy optimization of the isolated dopamine molecule and the hybrid dopamine−Ti2O4 in their ground electronic state. Optimized dopamine and hybrid structure are displayed in the inset of Figure 1, together with their respective HOMO and LUMO orbitals. It is apparent in this figure that the molecular HOMO is delocalized along the molecule, while the LUMO is mainly localized on the aromatic ring. For the hybrid system dopamine−Ti2O4, we note an important change of the charge localization in the HOMO, indicating a charge transfer from the molecule to the TiO2 cluster. Furthermore, the LUMO is mainly localized on the Ti2O4 cluster, a fact that supports the suggestion in ref 26 that, upon excitation with the laser used to probe the 1197



Article

Figure 2. Raman spectra for the dopamine molecule and hybrid system dopamine−Ti2O4 as a function of frequency shift (enlarged view of Figure 1 with a scaling factor of ×20 for the Raman signal of the isolated dopamine). Blue arrows indicate internal coordinates of the vibration involved in each normal mode. Inset: solid line, same as for the main figure; treen, dotted-dashed line, Raman activity of the deformed dopamine molecule for d = 1.3 Å.

and M3″ with frequencies 1314 and 1324 cm−1 in the hybrid. By way of comparison, we observe that mode M2 is the most enhanced (by a factor of 275) and mode M3 is the least enhanced one (by a factor of 29) by the presence of Ti2O4 cluster. These three modes involve mainly combination of the aromatic ring vibration and bending of the CH and/or OH bonds.

Raman response, there occurs an additional charge transfer, between the molecule and the nanoparticle, that is mostly responsible for the increase in the polarizability and hence for the Raman scattering enhancement. Indeed, it is clear that in the hybrid system, the electronic polarizability will show a very marked dependence upon small variation of the molecule− nanoparticule distance. Therefore, the polarizability gradient with respect to vibration modes, which the magnitude the Raman response is directly related to and where interfacial motion is involved, will increase in the hybrid system with respect to the isolated molecule. Figure 1 displays the Raman spectra of the molecule and hybrid system. We note that the Raman active modes can be divided into slow and fast modes and that they are separated by a frequency gap. The slow modes (frequency lower than 1.8 × 103 cm−1) involve the heavier atoms motion only, like CC, CC, CO, or CN vibrations as opposed to the fast modes (frequency higher than 2.8 × 103 cm−1) where H atom stretching motion is involved. In Figure 1, the high frequency modes mainly correspond to the OH stretching vibration. Comparing the Raman spectra of both the isolated molecule and hybrid, in Figure 1, we see that it is primarily the slow modes of the dopamine molecule that are enhanced by the presence of the Ti2O4 cluster. Among these slow modes, we have identified three modes of the isolated molecule that display a clear enhancement of the Raman activity induced by the coupling with the Ti2O4 cluster. An enlarged view of the Raman spectrum, for the molecule and hybrid system, centered on the frequencies of these three modes, is shown in Figure 2. The mode with a frequency of 1230 cm−1 in the dopamine molecule labeled as M1 can be associated with the mode labeled as M1″ with a frequency 1250 cm−1 in the hybrid. Modes M2 and M3 with frequencies 1283 and 1327 cm−1, respectively, in the molecule can be associated with modes M2″

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DEFORMED DOPAMINE We now turn to the main purpose of the present study, the Raman spectrum of the deformed isolated dopamine molecule. In the optimized structure of the isolated molecule, the O−H bond, indicated in the inset of Figure 1 has a length of d = 0.97 Å. As claimed in ref 30, this oxygen atom is the most acidic (Bronsted−Lowry) site of the molecule in aqueos and lipidic environments. The deformation we are going to consider is a systematic change in this O−H bond distance in the range d ∈ [0.95 Å, 1.50 Å]. For each value of d in that range, the electronic structures and the Raman spectrum are then calculated without reoptimizing the geometry. We limit ourselves to the study of the three modes described in the previous section, which are present in both systems (the deformed dopamine and the hybrid). In Figure 2, the inset shows how the frequency of each mode changes with O−H bond elongation. The modes in the deformed molecule are labeled as M1′, M2′, and M3′ to stress the correspondence with the modes of the optimized molecule. As the O−H motion does not participate in mode M3′, the frequency of this mode is almost not affected by the O−H deformation. The frequencies of modes M1′ and M2′ change slightly (less than 10%) as the O−H distance is varied. It is important to notice that the atomic displacements involved in mode M2′ are similar to those in the hybrid system mode M2″ but they are different from those in mode M2 corresponding to the optimized dopamine molecule. This can be rationalized in 1198



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Figure 3. Raman activity as a function of the O−H distance for the three normal modes in dopamine. Inset: normal-mode frequency as a function of O− H bond distance (d) for each normal mode of the dopamine molecule. Blue arrows indicate internal coordinates vibration involved in each normal mode.

density localization descriptor Ck(d) for the HOMO and LUMO, presented hereafter.

terms of the changes in the normal mode decomposition of the Raman modes for each system: the molecular mode M2 is suddenly turned into a mode that resembles a mode corresponding to the hybrid. This turnover behavior is one of the most interesting aspects of this study and is at the origin of our notation that associates modes M2, M2′, and M2″. The Raman activity of each mode, as a function of d is displayed in Figure 3. We note a remarkable behavior of the modes M1′ and M2′: their Raman activities drop to a very small value (less than 1 Å4 /amu) at d = 1.23 Å and then increase by a factor of 1.8 and 18.7, respectively, when the O−H distance increases by 5% (from d = 1.23 to d = 1.30 Å). On the contrary, the Raman activity of mode M3′ shows only small variations, which is consistent with the fact that the O−H motion is not involved in this mode. The Raman activity of the deformed dopamine molecule for d = 1.30 Å, as a function of the Raman shift frequency, is compared to the Raman activity of the optimized molecule in the inset of Figure 2. As shown in this inset, the Raman enhancement of the deformed molecule, mimics the Raman enhancement of the hybrid system. A quantitative comparison of all relevant intensities from this figure is given in Table 1. To understand why the Raman activities of modes M2′ and M3′ behave in such a singular way, we have calculated the orbital

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ORBITAL LOCALIZATION INDICATOR To quantify the variation, upon O−H elongation, of charge localization properties corresponding to any orbital ϕ(k)(r,⃗ d), we can use the first moment μ⃗(k)(d) of the orbital density ρ(k)(r,⃗ d) ≡ ∥ϕ(k)(r,⃗ d)∥2 corresponding to the molecular orbital ϕ(k)(r,⃗ d) defined as μ ⃗(k) (d) =

C(k)(d) ≡

=

mode no. 1

2

3

0.13 2.48 35.69 18.7 118.9

0.54 0.62 16.39 1.1 30.3

(1)

|| μ ⃗(k) (d)|| Q(k)(d)|| rM⃗ (d)|| (μx(k)(d))2 + (μy(k)(d))2 + (μz(k)(d))2 Q(k)(d)|| rM⃗ (d)||

(2)

where Q (d) = ∫ ) ρ (r,⃗ d) dr ⃗ and ∥rM ⃗ (d)∥ is the spatial extension of ) (i.e., the maximal length of r ⃗ in ) ). The orbital localization indicator C(k)(d) lies between 0 and 1 and is invariant under molecular rotation. C(k)(d) = 0 means that all the orbital density is concentrated at the origin of coordinate and C(k)(d) = 1 corresponding a distribution located on the surface of the box ) . In Figure 4, C(HOMO)(d) and C(LUMO)(d) as a function of d are shown. We see that C(HOMO)(d) remains almost constant when d ∈ [0.8 Å, 1.5 Å], but C(LUMO)(d) suffers an abrupt change at exactly d = 1.23 Å, the precise distance where the Raman activity behaves singularly. Indeed, C(LUMO)(d) decreases by almost 30% when d increases by less than 0.01 Å. In the same figure, some (k)

Table 1. Raman Activities (Å /amu) and Factor of Enhancement at d = 1.30 Å

0.90 1.59 2.29 1.8 2.5

3

where r ⃗ denotes the electronic coordinates, d is the O−H distance, and ) is a box embedding the chosen orbital. The norm of μ⃗ (k) suitably normalized will give a direct indication of the change of spatial charge distribution. For this purpose, for each molecular orbital, we define

4

M M′ M″ M′/M M″/M

∫)⊂ ρ(k)( r ⃗ ,d) r ⃗ d r ⃗

1199

(k)



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Figure 4. Orbital density localization indicator C(k)(d) for the HOMO and LUMO of dopamine as a function of the O−H distance (d). Blue rhombus: HOMO. Red square: LUMO. Snapshots of the HOMO (bottom) and LUMO (top) electron density of dopamine, for the O−H distances indicated by the arrows. Inset: enlarged plot where the abrupt change takes place.

Figure 5. Orbital energies as a function of O−H distance (d). Red squares: LUMO+1. Blue rhombus: LUMO. Green triangles: HOMO.

snapshots of the corresponding electronic density distribution

This very singular behavior of the orbital density distribution is in fact the result of an energy crossing between the LUMO and the second unoccupied molecular orbital LUMO+1, as shown in Figure 5. In this figure the orbital energies of the HOMO, LUMO, and LUMO+1 as a function of d are displayed. We can appreciate that at the value d = 1.23 Å the first two unoccupied molecular orbitals are degenerated in energy.

are also presented. They clearly show a dramatic change in the charge distribution occurring at d = 1.23 Å. The density that is distributed on the aromatic ring for d < 1.23 Å becomes suddendly localized near the O−H bound when d reaches the value d = 1.23 Å. 1200



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Figure 6. Components of the polarizability tensor (in Debye units) as a function of d (Å).

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A different and complementary perspective of the physical process involved is afforded by the description of the gradient of the polarizability with respect to d (Figure 6). As is well-known, this is the magnitude that enters in the calculation of the nonresonant Raman response and understanding its behavior is a key ingredient in the analysis of the physical model. This is why we observe that at d = 1.23 Å, the slope of the curves present a sudden decrease, corresponding to a decrease of the polarizability gradient.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. ¶ Institut Universitaire de France.

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ACKNOWLEDGMENTS We acknowledge financial support from the ANR-11-NS04-0001 FRAMOLSENT program. This work was performed using HPC resources from GENCI- CINES/IDRIS (Grant 2012x2012082131, 2013- x2013082131) and the CCRE-DSI of Université P. M. Curie.

DISCUSSION AND CONCLUSION

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Conceptually, the O−H bond distance can be thought as a generalized reaction coordinate where the molecular energy depends on a controlable parameter that physically represents a deformation of the molecular geometry, induced by the formation of a bond between the molecule and the oxide cluster. This model is substantially rewarding in that we have shown a remarkable correlation between the Raman enhancement observed in the hybrid and the Raman response of a fictitious molecular structure, that of the deformed dopamine. Given the virtual nature of the photon absorption involved in Raman scattering, it comes as a natural result that the turnover region of the Raman activity as a function of the control parameter is correlated with a crossing of unoccupied states that would take part in the resonant virtual excitation involved in the Raman response. In fact, we have shown that the O−H deformation induces an energy crossing between two virtual orbitals of the dopamine molecule, which dramatically changes the Raman cross sections. This crossing induces a singular behavior of the polarizability gradient, which in turn changes the Raman activity of the modes where the O−H deformation is involved, resulting in a Raman activity enhancement. This behavior qualitatively mimics the change in virtual orbitals and Raman enhancement induced by the chemisorption of the molecule on a Ti2O4 cluster. At the DFT level of calculations, we have thoroughly accounted for some chemical processes that may be responsible for the Raman signal enhancement of dopamine−Ti2O4 hybrids. By their expected general applicability, such processes should presumably be transposable to other organic molecules on semiconductor nanoparticles.

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Probing Raman Enhancement in a DopamineâTi2O4 Hybrid Using

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