Theoretical Study of the Surface Complex between TiO2 and TCNQ

LETTER pubs.acs.org/JPCL

Theoretical Study of the Surface Complex between TiO2 and TCNQ Showing Interfacial Charge-Transfer Transitions Ryota Jono,† Jun-ichi Fujisawa,‡ Hiroshi Segawa,‡ and Koichi Yamashita*,† †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan

bS Supporting Information ABSTRACT: The surface complex of TiO2 nanoparticles and TCNQ was studied using density functional theory (DFT) calculations. The structure of the surface complex was optimized, showing an IR spectrum analogous to the experimental spectrum. From time-dependent DFT calculations based on this optimized structure, we demonstrated that the interfacial charge-transfer transitions from the HOMO of the surface-bound TCNQ molecule to the unoccupied levels of the TiO2 nanocluster occur in the visible to near-IR region. SECTION: Molecular Structure, Quantum Chemistry, General Theory

P

hotoinduced charge separation at the surface between two materials is a key process for photoenergy conversion systems.1 In almost all photovoltaic devices, photoinduced charge separation occurs via light absorption by composite materials and subsequent charge separation at the interface. In contrast to this mechanism, charge separation can take place directly by charge-transfer transitions at the interface. Because of the nature of the one-step charge separation, interfacial charge transfer transitions are expected to be useful for efficient photoelectric conversion reactions. Many researchers have been attracted to this area,1
3 but an efficient photoelectric conversion based on this mechanism has not yet been reported. Recently, we reported a novel material for this mechanism4 and briefly review this material in the following paragraph. TiO2 (anatase) nanopowder is colored violet by immersion in the solution of 7,7,8,8-tetracyanoquinodimenthane (TCNQ), although both the materials do not show absorption in the visible region, which is indicative of the formation of a surface complex. Figure 1a shows the diffuse reflectance spectrum of the surface complex. The broad absorption band in the visible to nearinfrared (IR) region was attributed to interfacial charge-transfer transitions from the TCNQ adsorbate to the TiO2. The FT-IR spectrum was observed to reveal the surface complex structure, and the spectrum in the range from 2050 to 2300 cm
1 is shown in Figure 1b. The three peaks in Figure 1b that appear upon adsorption of TCNQ onto TiO2 are attributed to CN stretching modes, in contrast to a single mode observed for TCNQ. These three peaks were similar to those of the methoxy TCNQ CH3O
TCNQ
.5 r 2011 American Chemical Society

Therefore, we have experimentally revealed that a surface complex is formed by the nucleophilic reaction of TCNQ with the surface hydroxyl groups of the TiO2 nanoparticles, generating a negatively charged TCNQ adsorbate (Figure 1b). However, this model has not been confirmed theoretically yet. It is important to study this novel material theoretically to understand the structure and electronic transitions. In this Letter, we report on the structure, electronic states, vibrational structure, and electronic transitions of the TiO2
TCNQ surface complex calculated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. In our calculations on the surface complex, we began by modeling anatase TiO2 with a Ti9O18H
OH nanocluster, which has a hydroxyl group that is able to form a bonding site for TCNQ and a proton at the surface for charge neutralization. This nanocluster was optimized at the B3LYP/6-31G* level of theory using the Gaussian09 software package,6 constraining some angles and dihedral angles to their experimental values.7 The conductorlike polarizable continuum model (CPCM)8 was used to model the acetonitrile solution, which was used as an immersion solvent in our experiments. All of the optimization procedures for the TiO2 moiety were performed according to this computational scheme. Figure 2a shows the density of states (DOS) of the Ti9O18H
OH nanocluster in acetonitrile. The HOMO and LUMO of Ti9O18H
OH were comprised of oxygen 2p and Received: March 22, 2011 Accepted: April 19, 2011 Published: April 28, 2011 1167

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LETTER

Figure 3. Calculated vibrational spectrum of the Ti9O18H
O
TCNQ
surface complex in acetonitrile around the CN stretching modes. The inset shows each CN vibrational mode.

Figure 1. (a) Experimentally observed diffuse reflectance spectra of the TiO2 and TiO2-TCNQ nanopowders and absorption spectrum of TCNQ in acetonitrile. (b) Experimentally observed FT-IR spectra of TCNQ and the TiO2
TCNQ nanopowder together with the experimentally proposed structure of the TiO2-TCNQ surface complex.4

Figure 4. Calculated absorption spectrum of the Ti9O18H
O
TCNQ
surface complex in acetonitrile. The red lines denote charge-transfer transitions, and the blue lines denote electronic transitions in the TiO2 nanocluster.

Figure 2. Total and partial density of states of (a) Ti9O18H
OH anatase and (b) Ti9O18H
O
TCNQ
surface complex with the optimized structures in acetonitrile. Key: silver = Ti, red = O, gray = C, blue = N, and white = H.

titanium 3d orbitals, respectively. These energies correspond approximately to the valence band edge and conduction band edge of bulk TiO2.9 The DOS was also very similar to that of Ti9O18H
OH calculated by using the PBE0 functional or aug-cc-pVDZ basis set and that of larger cluster Ti25O50 (Figure S1 in Supporting Information). Therefore, we can regard this nanocluster as a good model structure reproducing optical properties of TiO2. Finally, the complex Ti9O18H
O
TCNQ
in acetonitrile that corresponds to the structure indicated by our experiment was optimized at the B3LYP/6-31G* level of theory under Cs

symmetry. The optimized structure and its HOMO and LUMO are shown in Figure 2b and Figure S2 in Supporting Information, respectively, and its Cartesian coordinates are listed in the Supporting Information. The optimized Ti9O18H
O
TCNQ
gives a similar TCNQ moiety structure to the experimentally suggested structure (Figure 1b). Figure 2b shows the DOS and the optimized structure of Ti9O18H
O
TCNQ
in acetonitrile. The data in Figure 2b show that the HOMO level of the surface-bound TCNQ molecule, which has a negative charge is in the band gap of the TiO2 cluster. The polar acetonitrile solvent tends to stabilize this electronic structure. The vibrational spectrum was examined to justify the structure of the surface-bound TCNQ molecule. The calculated vibrational spectrum from the four CN stretching modes consists of three resolved peaks because the positions of ν3 and ν4 were very close, as shown in Figure 3. These characterizations are also shown in Figure 3. This vibrational spectrum is analogous to the experimental FT-IR spectrum (Figure 1b), using an energy scaling factor of 0.9613.10 This result supports the structure of the surface-bound TCNQ molecule. On the basis of the optimized structure, the excitation spectrum of Ti9O18H
O
TCNQ
in acetonitrile was calculated using the TD-DFT method, as shown in Figure 4. The excitation characteristics of Ti9O18H
O
TCNQ
are listed in Table 1. Figure 4 indicates that novel excitation bands appear in the visible to near-IR region for the surface complex. From Table 1 and Figure 4, it can be seen that these excitations arise 1168

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Table 1. Excitation Characteristics of Ti9O18H
O
TCNQ
excitation energy/nm

charactera,b (|c| > 0.25)

oscillator strength

TiO2 ratio of MOc

1165.28

0.0001

0.7 H f L

CT

1f99%

886.10

0.0012

0.7 H f L þ 1

CT

1f99%

700.53

0.0001

0.7 H f L þ 2

CT

1f99%

693.20

0.0272

0.7 H f L þ 3

CT

1f96%

634.01

0.0195

0.68 H f L þ 6

CT

1f92%

542.83

0.0591

0.68 H f L þ 12

CT

1f84%

467.11

0.0659

0.69 H f L þ 16

CT

1f94%

430.26

0.0321

0.52 H f L þ 19
0.42 H f L þ 21

CT CT

1f94% 1f89%

406.41

0.0578

0.64 H f L þ 22

CT

1f85%

392.59

0.1385

0.46 H f L þ 24

CT

1f72%

0.49 H f L þ 26

CT

1f37%

0.28 H
2 f L þ 6

LE

100f92%

0.37 H
2 f L þ 9

LE

100f100%


0.27 H
1 f L þ 4

LE

99f100%

351.16

0.0197

a

The H and L in the character column denote the HOMO and LUMO, respectively. b CT and LE denote charge-transfer transitions and local excitations in TiO2, respectively. c The TiO2 ratio was calculated as the ratio of the atomic orbitals of Ti9O18H
O in each MO.

from interfacial charge-transfer transitions from the negatively charged TCNQ adsorbate to the unoccupied levels of the TiO2 nanocluster. Furthermore, the data indicate that the experimentally observed broad absorption band consists of several interfacial charge transfer transitions, which excite an electron in the TCNQ moiety to the different electronic states in TiO2. Thus, for those excitations, electron dynamics such as a recombination process is probably different. This information is helpful in gaining an understanding of the electron dynamics in solar cells using the TiO2
TCNQ surface complex and is also important for improving the energy conversion efficiency.4 Further studies on the interfacial charge-transfer transitions are now in progress. In conclusion, we have studied the TiO2
TCNQ surface complex theoretically. The structure of the surface complex was optimized using DFT calculations. On the basis of the optimized structure, our TD-DFT calculations successfully modeled the experimentally observed interfacial chargetransfer transitions in the TiO2
TCNQ surface complex for the first time.

’ ASSOCIATED CONTENT

bS

Supporting Information. Cartesian coordinate of the optimized structure and reference calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Funding Program for WorldLeading Innovative R&D on Science and Technology (FIRST) “Development of Organic Photovoltaics toward a Low-Carbon Society,” Cabinet Office, Japan.

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