Adsorption of Dye Molecules on Single Crystalline Semiconductor

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Adsorption of Dye Molecules on Single Crystalline Semiconductor Surfaces: An Electrochemical ShellIsolated Nanoparticle Enhanced Raman Spectroscopy Study Liqiang Xie, Ding Ding, Meng Zhang, Shu Chen, Zhi Qiu, Jia-Wei Yan, Zhilin Yang, Mingshu Chen, Bing-Wei Mao, and Zhong-Qun Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07763 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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The Journal of Physical Chemistry

Adsorption of Dye Molecules on Single Crystalline Semiconductor Surfaces: An Electrochemical Shell-isolated Nanoparticle Enhanced Raman Spectroscopy Study †

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‡

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Li-Qiang Xie , Ding Ding , Meng Zhang , Shu Chen , Zhi Qiu , Jia-Wei Yan , Zhi-Lin Yang , †* † † Ming-Shu Chen , Bing-Wei Mao *, Zhong-Qun Tian †

State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of

Chemistry and Chemical Engineering, iChEM, Xiamen University, Xiamen 361005 (China). ‡

Department of Physics, Xiamen University, Xiamen 361005, China

*Corresponding authors. E-mails and telephone numbers: [email protected], +8605922186862; [email protected], +8605922183723

ABSTRACT Adsorption of dye molecules on semiconductor surfaces dictates the interaction at and thus the electron transfer across the interface, which is a crucial issue in dye-sensitized solar cells (DSSCs). However, despite that surface enhanced Raman spectroscopy (SERS) has been employed to study the interface, information obtained so far is gathered from surfaces of irregularly arranged nanoparticles, which places complexities for precise attribution of adsorption configuration of dye molecules. Herein, we employ single crystalline rutile TiO2(110) for Raman spectroscopic investigation of TiO2-dye interfaces under electrochemical control by utilizing the enhancement of Au@SiO 2 core-shell nanoparticles. FD-TD simulation is performed to evaluate the localized electromagnetic field (EM) created by the core-shell nanoparticles while Mott-schottky measurements to determine the band structure of the semiconductor electrode. Comparative investigations are carried out on nanoporous P25 TiO2 electrodes. Potential-dependent Raman shift of (N=C=S) suggests that the binding of SCN group of N719 to the TiO2 surface is the 1

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intrinsic nature of TiO2-N719 interaction, after removing the possible bonding complexity by surface roughness. Nevertheless, hydrogen bonding between COOH and the TiO2 appears to be more favorable on atomic flat rutile TiO2(110) surface than on the surface of nanoporous P25 nanoparticle as revealed by stronger Raman shift of (C=O) (COOH) on the former. Electrochemical SERS (EC-SERS) results show that photo-induced charge transfer (PICT) occurs for both the P25 and rutile(110) TiO2 surfaces and the potential to achieve PICT resonance depends on the band structure of the semiconductor. Our work demonstrates that EC-SERS can be applied to study the single crystalline semiconductor-molecule interfaces using core-shell based surface plasmonic resonance (SPR) enhancement strategy, which would promote fundamental investigations on interfaces of photovoltaic and photocatalytic systems.

INTRODUCTION Dye-sensitized solar cells (DSSCs) have attracted much interest over the past twenty years because of their low cost, long durability, high efficiency as well as the potential as a more environmentally friendly alternative to traditional solar cells1. In these cells, dye molecules are adsorbed on the surface of a wide band-gap semiconductor, typically TiO2, which themselves absorb the light solely in the high energy ultraviolet region. The dyes in such cells play a crucial role, as they act as antennas to harvest solar light and give rise to charge separation by injecting electrons into the conduction band of the TiO 2. Along with the development of DSSCs, an enormous number of dye molecules have been developed with main focuses on improving their molar absorption coefficient in the visible and near-infrared wavelength range or adjusting their HOMO and LUMO energy levels to fit the band of TiO2 to enhance cell performance2. To achieve high quantum yields of the electron transfer process, ideally the dye needs to be intimately in contact with the TiO2 surface. Thus the solar energy conversion efficiency and cell performance are highly dependent on the type of surface-anchoring3. 2


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A number of techniques, including UV-vis absorption spectroscopy4-7, infrared spectroscopy3, 5, 7-12, Raman spectroscopy4-6, 9-11, 13-19, photoelectron spectroscopy20, and XPS21

have

been

used

to

study

the

interfacial

structure

of

N719

(bis

(tetrabutylammonium)[cis-di(thiocyanato)-bis(2,2’-bipyridyl-4-carboxylate-4’-carboxylic acid)-ruthenium(II)])) dye molecules adsorbed onto nanoporous TiO2. Structural and electronic properties of the N719-TiO2 interface have been studied by theoretical calculations based on first principle methods22-25. Falaras et al8 reported based on FTIR spectrum that the N719 sensitizer anchors to TiO2 surface via an ester like bonding which favors

high

photovoltaic

performance.

Finnie

et

al9

suggest

that

N3

(cis-di-(thiocyanato)-N,N-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium (II)) sensitizer binds to TiO2 surface via chelating or bridging type binding, which can be deduced

from

the

splitting

of

carboxylate

stretching

bands,

i.e.

=asym(COO-)-sym(COO-). The chelating or bridging type binding together with H-bonding of carboxylate group onto TiO2 surface is later agreed by many other researchers based on experimental5,

7, 10

and theoretical23 investigations. Furthermore,

Johansson et al20 found by photoelectron spectroscopy study that the NCS group on the dye molecule can also interact with TiO2 surface through the sulfur atom. Recently, Lee et al21 reported that electronic interactions between N719 and TiO2 occurs not only through the covalent bonding of the anchoring groups, but also through the aromatic electron of the bipyridine groups with the d states in TiO2 which was revealed by the results of extended X-ray absorption fine structure measurements. Although a lot of efforts have been devoted to clarify the type of binding for the complex yet practically and fundamentally important N719-TiO2 interface, the detailed and exact interfacial structural configuration and its influence on the charge transfer process are still not clear. In our previous work, we employed Ag@TiO216 and dimeric Ag2@TiO217 core-shell nanoparticles in which the shells can provide TiO2 working surfaces for EC-SERS investigations of N719 adsorbed on TiO2. By taking advantage of the surface plasmonic 3



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resonance (SPR) of the Ag core, a surface enhancement factor up to 1010 was achieved, which allowed for pre-resonance (excited by 638 nm laser) and non-resonance (excited by 785 nm laser) Raman investigation under electrochemical control, while the thin TiO2 shell itself contributed little Raman signals. The potential dependency of the Raman spectral features observed with 638 and 785 nm excitations discloses that the adsorbed N719 dye molecules interact with the TiO2 surface through COO- and SCN groups while photo-induced charge transfer (PICT) process depends on the excitation energy of light. However, for a molecule having multi-functional groups such as N719, its adsorption onto nanoporous electrodes such as TiO2 and Ag@TiO2 can result in the complexity of adsorption of different functional groups onto neighboring nanoparticles because of steric effect, which hinders one from extracting the intrinsic nature of the interfacial interactions. In this work, we employ rutile(110) single crystalline substrate of TiO2 for investigation of adsorption configuration and conformational change of N719 by Raman spectroscopy. Rutile(110) is the most stable single crystalline surface of TiO2. This type of crystals is well-characterized and easy to grow and polish to obtain surfaces of high quality. Since Raman signals of adsorbates on single crystalline surface are extremely low, Au@SiO2 core-shell nanoparticles are used to enhance the electromagnetic field and thus the Raman signals. The method is termed as shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS), which was established and developed by Tian group26-28. FD-TD simulation is carried out to reveal the location of the hot spot and to estimate the electromagnetic field enhancement at the hot spot. For comparison, SHINERS study of N719 adsorbed on P25 nanoporous film electrode is also carried out. Mott-schottky measurements for flat band potentials of the two systems are obtained by means of electrochemical impedance spectroscopy to reveal the PICT mechanism. These results demonstrate, for the first time, that single crystalline TiO2 surfaces can be studied by electrochemical SHINERS (EC-SHINERS), and this would promote spectroscopic 4


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characterization of photovoltaic and photo catalytic interfaces with well-defined structures.

EXPERIMENTAL Preparation of Rutile (110) The rutile (110) 550.5 mm3 cube sample (Hefei Ke Jing materials technology co., LTD.) was used. To obtain a smooth surface with improved electronic conductivity in its bulk for electrochemical measurements, argon sputtering followed by electron beam bombardment annealing in vacuum was applied. The pressure of our vacuum system reaches 6.010-9 Torr, and the preparation follows repeated cycles of ion sputtering for 15 min (1000 eV Ar+), annealing to 900 K and then maintaining

for 30 min. After the

sputtering-annealing process the sample exhibited dark blue color and then was transferred to an isolated chamber in which the rutile (110) was quickly immersed in an ethanol solution containing 0.3 mM N719 under N2 atmosphere. After 12 h the rutile (110)/N719 was rinsed with ethanol for 3 times until the ethanol has no obvious color change in order to remove physically adsorbed N719 molecules. The rutile(110)/N719 electrode was ready for use for SHINERS experiments. Preparation of thin film electrode The rutil P25 TiO2 nanoparticles were purchased from Evonik Degussa corporation in Germany. The nanoparticles were dispersed in a solvent of 1:1 ethanol:H2O and then centrifuged at 7500 r/min for 15 min. The P25 nanoparticles after purification were dispersed again and then immobilized onto glassy carbon (GC) electrode. The GC was then annealed at 192 ℃ for 1 h. The morphology of the P25 thin film electrode was characterized by scanning electron microscope as shown in Figure S4. Synthesis and Characterization of Au@SiO2 NPs Au@SiO2 nanoparticles were synthesized using the method reported by Li et al26-27 after modification. Au spherical nanoparticles with diameter of 55 nm were prepared using the 5



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sodium citrate reduction method29. 200 mL HAuCl4 aqueous solution (0.01 weight %) was heated to boil. Then 1.6 mL sodium citrate aqueous solution (1 weight %) was added to the HAuCl4 solution. The color of the boiling solution changed from transparent to dark in a few minutes and then to wine. The diameter of the Au NPs can be easily tuned by adjusting the volume of the reducing agent, i.e., sodium citrate aqueous solution. The ultra-thin silica shell was coated by the high temperature silicate hydrolysation method30-31. 30 mL of Au NPs solution was placed in a 100 mL round-bottom flask and stirred for 10 min. 404 L of (3-aminopropyl)trimethoxysilane aqueous solution (1 mM) was added to the NPs solution and stirred for another 15 min at room temperature. Sodium silicate solution with pH adjusted to about 10.0 by sulfuric acid was added to the flask and stirred for another 5 min. Then the mixture was heated to boil for 45 min in a boiling water bath to obtain pinhole free Au@SiO2 NPs with ca. 3 nm silica shell thickness. The overall morphology of Au@SiO2 core-shell NPs were characterized by SEM and high resolution TEM, as shown in Fig. 1B and 1C. SERS experiments on Au@SiO2/pyridine interfaces were carried out to make sure no pinholes left on the SiO2 shell and no pyridine molecules adsorb on the surface of the Au core.

Electrochemical Raman measurements Raman measurements were conducted on XPLORA Raman spectroscopy (Jobin-Yvon, France) with excitation line of 638 nm. Laser power density is ca. 1.5 mW. Optical grating of 1200 T was used. The EC-SHINERS experiments were conducted in a three-electrode electrochemical Raman cell in which Pt wires served as both the reference and counter electrodes. A solution of acetonitrile

containing 10 mM LiClO4 was used as

control solution for electrochemical measurements.

Mott-Schottky measurements Mott-Schottky measurements of rutile(110) and P25 TiO2 electrodes in CH3CN 6


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containing 10 mM LiClO4 were conducted on a PSGSTAT128N autolab electrochemical workstation by means of Frequency Response Analysis-Potential Scan technique.

Figure 1.

(A) Schematic illustration of SHINERS configuration for investigation of N719 adsorbed

on TiO2 rutile(110) single crystal electrode, (B) SEM image of a monolayer Au@SiO2 nanoparticles assembled on silicon surface, (C) TEM image of a typical Au@SiO2 core-shell nanoparticle showing no pinholes on the ultrathin silica shell, and (D) FD-TD simulated EM distribution at the surface of rutile (110) single crystal, which is created by two closely located Au@SiO2 nanoparticles in coupling with the rutil(110) surface. The simulation in (D) is for the rutile (110) with improved conductivity as obtained by Ar+ sputtering and annealing.

RESULTS AND DISICUSSION Electromagnetic field enhancement (EM) is considered to be the dominant contribution 7



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of SERS effect. However, for most semiconductor materials, the SPR locates in the infrared region which is far from the visible region we normally work with. Therefore, the SPR from the titanium dioxide itself contributes little to the overall SERS enhancement. Instead, it was reported that TiO2-to-molecule charge-transfer may be the dominant mechanism of SERS for systems such as 4-mercaptobenzoic acid adsorbed on TiO2 nanoparticles excited by a 514.5 nm laser32. On the other hand, for N719 dye molecules whose molecular structure is shown in Figure 2A,

electron transfer occurs

under light excitation from the highest occupied molecular orbital (HOMO), shared by Ru and SCN ligands, to the lowest unoccupied molecular orbitals (LUMO), concentrated on the π* structure of the ligand33, namely metal to ligand charge transfer (MLCT） process. Figure 2B is the UV-vis absorption spectrum of N719 dissolved in ethanol, two peaks, both related to MLCT, are present at 532 and 387 nm, respectively. The lower energy 532 nm peak which extends to 700 nm may lead to resonance Raman process under laser excitation near 532 nm. Noteworthy is that Raman signals of monolayer N719 molecules adsorbed on atomically flat TiO2 single crystalline surface are normally under the detection limit of most Raman instruments. Even for P25 nanoporous TiO2 substrate whose specific surface area is orders of magnitude larger so that much more N719 molecules can be adsorbed, Raman spectra of the molecules are usually obtained with the aid of resonance Raman process of N719. 14 A comparison of Raman spectra from rutile TiO2(110) surfaces adsorbed with N719 under 532 and 638 nm laser excitations are provided in Figure 2C. Under 532 nm laser excitation, bands from the adsorbed N719 show up at 1468, 1534 and 1607 cm-1 (see inset of Figure 2), which are resonance Raman signals associated with the stretching of C=C and C=N on the bipyridine rings. Unfortunately, these signals conceal the normal Raman signals of other bands of the adsorbed molecule which could be valuable in elucidating the interaction at the interface. On the other hand, under the laser excitation of 638 nm that locates at the absorption tail of the lower energy side, the pre-resonance Raman 8


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spectrum show only three peeks from the bulk rutile titania, i.e., the multi-phonon

Figure 2. (A) molecular structure of N719, (B) UV-vis absorption spectrum of N719 in ethanol, and (C) SHINERS spectra of N719 adsorbed on rutile (110).

process at 235 cm-1, Eg mode at 443 cm-1 and A1g mode at 613 cm-1.34

No signals from

the molecules are detectable, which implies that the TiO2-molecule charge-transfer mechanism alone, if any, cannot create sufficiently strong Raman signals in the present system, unlike the system of 4-mercaptobenzoic acid adsorbed on TiO2 nanoparticles. To investigate N719 dye molecules adsorbed on single crystalline rutile (110) surfaces of TiO2 by Raman spectroscopy under pre- or off-resonance condition, core-shell strategy is adopted by assembling a few layers of Au@SiO2 core-shell NPs with 55 nm Au core and 3 nm silica shell on top of the N719 monolayer. Here the NPs are isolated from N719 molecules and are expected to enhance the Raman signals of N719 molecules without direct chemical interactions with the molecules. The pre-resonance SHINERS results under 638 nm laser excitation are shown in wine in Figure 2C, and several features are noteworthy: 1) Signals from the bulk TiO2 appear very weak because the incident laser is focused on top of Au@SiO2 layers. 2) The Raman signals of the N719 molecules in the 9



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region between 900 and 2200 cm-1 are much stronger compared with those from the bulk TiO2. The Raman spectrum is dominated by the ring breathing mode at 1028 cm-1 as well as stretching of C=C and C=N on the bipyridine rings at 1269, 1478, 1551 and 1611 cm-1. 3) Moreover, the vibrational bands from the ligands are clearly observed, including C=O vibrational mode of carboxyl group at 1736 cm-1, C=N vibrational mode of thiocyanate at 2136 cm-1 and C-C inter ring vibrational mode at 1324 cm-1. To understand the SERS effect from the N719/TiO2 rutile(110) interface, the EM enhancement arising from the TiO2 rutile (110)/Au@SiO2 coupling is simulated by FD-TD calculation, which demonstrates that the hot spots locate at the Au@SiO2-rutile (110) gaps. The dielectric function used in the FD-TD simulation here is measured by ellipsometer and the data is given in supporting information. The simulated surface enhancement factor arising from the SPR created by two closely located Au@SiO2 nanoparticles is about 2600. The overall SERS enhancement is suggested to be a combination of EM enhancement by Au@SiO2, charge transfer and pre-resonance effect. The employment of the TiO2 single crystal removes the complexity of adsorption of different functional groups from the same N719 molecule onto neighboring particles of TiO2 because of steric effect. Therefore, the SHINERS can favorably provide the intrinsic information about the interaction of N719 with TiO2 at the interface. To understand in more details about the N719 adsorption on TiO2 as well as the PICT mechanism, we performed potential-dependent SHINERS experiments on well-defined TiO2 rutile(110) single crystal electrode. We mention that the bulk resistance of a TiO2 single crystal in the size of 5 mm5 mm0.5 mm is normally so large that obvious ohmic polarization makes it difficult for electrochemical investigations . In order to promote the bulk conductivity of rutile(110),

an argon sputtering-annealing method was used to create bulk oxygen

vacancies and so to increase charge carrier density. Such a method has been widely used to refresh TiO2 single crystalline surface for surface science studies in ultrahigh vacuum. After

sufficient 10


Ar+

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Figure 3. Electrochemical SHINERS spectra of N719 adsorbed on TiO2 rutile (110) single crystalline electrode. Asterisks indicate the signals from the bulk of TiO2.

sputtering-annealing cycles, the rutile (110) appears in dark blue with carrier density to a value on the order of 1017 to 1018 cm-3.35

Photo-electrochemical response of this single

crystalline rutile(110) electrode with N719 monolayer adsorbed is shown in supporting information together with P25 nanoporous electrode. The single crystalline TiO2 rutile(110) with high conductivity was then used as the electrode for SHINERS investigation of TiO2-N719 interface, and the obtained EC-SHINERS spectra are shown in Figure 3. The spectral features in the low frequency region are associated with, Eg mode at 443 cm-1 and A1g mode at 613 cm-1of rutile phase TiO2, except that the 380 cm-1 band is the asymmetric C–CN bending mode (CCN) from the acetonitrile solvent36. In the medium frequency region, Raman spectra at all measured potentials are dominated by vibrational bands related to the bipyridine ring, namely the ring breathing mode at 1028 cm-1, C=C vibrational mode at 1545 and 1612 cm-1, C=N vibrational mode at 1273 and1478 cm-1, and C-C inter ring vibrational mode at 1308 cm-1. Nevertheless, the bands 11



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at 1374 and 1734 cm-1 assigned to s (COO-) from the carboxylate group and C=O) from the carboxylic group (COOH), respectively, are still identifiable. Especially, for the s (COO-) band, a weak yet obvious potential-dependency of Raman intensity is observed, which indicates that the COO- group binds to TiO2 surface for the adsorption of N719. From the consideration of N719 molecular steric configuration, it is impossible for all of the two carboxylate and two carboxylic groups of the same molecule to anchor to the rutile (110) surface, see Figure 5A. Lee et al suggested only the two COO- groups of N719 are used to bind to TiO2 while the rest remain free10. Indeed, the emergence of the band at 1734 cm-1 in the present work indicates that some of the COOH are free of surface binding. While the carboxylic adsorption configuration is widely accepted, the adsorption of N719 does not solely rely on it. As can be seen from the high frequency region of the spectrum, the (C=N) (SCN) band at 2142 cm-1 at 0.15 V shifts to 2147 cm-1 at -0.45 V, then shift to 2141 cm-1 at -1.05 V. This indicates that the SCN group is directly bonded to rutile (110) surface as well, and we suggest this binding most likely occurs via hydrogen bonding between the sulfur atom of SCN and the hydroxyl group on the TiO2 surface. On the basis of the above-mentioned analysis, adsorption configuration of N719 on TiO2 rutile(110) is summarized and illustrated in Figure 5A. Briefly, on the TiO2 rutile(110) surface, the intrinsic adsorption configuration of N719 anchoring to TiO2 is via COO- and SCN with some of the COOH groups remaining free. For comparison, we also conducted pre-resonance electrochemical Raman measurements for N719 adsorbed on P25 nanoporous film electrode based on the Au@SiO2 core-shell strategy, and results are shown in Figure 4. A few similarities and differences in the spectral features are observed between the P25 nanoporous and TiO2 rutile (110) single crystal surfaces. First, in the low frequency region, bands at 515 and 641 cm-1 corresponding to A1g and Eg modes of anatase TiO2, respectively, are observed, which indicates that anatase is the dominant phase in P25 used in the present work. The asymmetric C–CN bending mode (CCN) of acetonitrile is exactly the same in 12


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Figure 4. Electrochemical SHINERS spectra of N719 adsorbed on P25 NP electrode.

wavenumber and intensity as those in the case of rutile(110) and shows no potential-dependency. Therefore, this band can serve as an internal reference of Raman shift and peak intensity for both P25 and rutile(110) systems and facilitate convenient comparisons of the two interfaces. Second, the potential-dependency of the Raman shift of the (C=N) (SCN) band at 2144 cm-1 is weaker compared with that on the rutile(110) single crystalline surface, implying weaker adsorption on the former than the latter. Nevertheless, the relative intensity of (C=N) (SCN) mode against the ring breathing mode at 1028 cm-1 as well as the asymmetric C–CN bending mode (CCN) of acetonitrile is stronger than that on the smooth rutile(110) surface. This indicates that most likely both the two thiocyanato groups on a N719 molecule are employed and bond to two neighboring P25 nanoparticles, respectively, while only one of the two thiocyanato groups is used to bind to rutile (110) surface. Third, the carboxylate symmetric vibrational mode s(COO-) on P25 (1379 cm-1) shows no potential dependency of 13



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intensity and Raman shift. However, for the (C=O) band of the free carboxylic group (COOH), it appears at 1742 cm-1 on the P25 nanoporous surface compared with 1734 cm-1 on the rutile (110) surface. According to Finnie et al.'s observation that the vibrational mode of C=O will shift to lower frequency if hydrogen bond forms between the carboxylic group and other species9, we suggest that hydrogen bonding is stronger on the atomically flat rutile (110) surface than on the nanoporous P25 film electrode. Fourth, the carboxylate symmetric vibrational mode s (COO-) at 1379 cm-1 on P25 is observed, compared with that at 1374 cm-1 on rutile (110), i.e. red shift occurs when N719 is adsorbed onto rutile(110). This means that carboxylate group is also essential for N719 dye to bond to TiO2 surface. Unfortunately, no asymmetric vibrational mode are observed in both systems, unlike in the IR study by Finnie and co-workers9. Based on the above-given analysis, the adsorption of N719 onto the P25 nanoporous surface is fulfilled somewhat through the interaction of the SCN, which then brings the COOH or COO groups closer to the surface for interaction. The adsorption configuration of N719 on rutile（110）and P25 nanoparticle surfaces of TiO2 are illustrated in Figure 5. Here we wish to mention that it has been shown in our previous work that under 638 nm excitation without the SPR enhancement by Au@SiO2 nanoparticles, the Raman signals associated with N719 molecule are far weaker than those from P25 substrate and information from the COOH and NCS groups are too weak to be detected even when the collection time reached as long as 100 seconds16. However, as shown in the present work, in the presence of a few layers of Au@SiO2 nanoparticles assembled on top of the N719 monolayer, the intensity of Raman signals arising from the molecules can be enhanced. The favorable SHINERS results from both the single crystal and P25 film electrodes lay foundation for comparative investigations on the practically important semiconductor surfaces with different structural ordering. For understanding the PICT contribution to the SERS of N719/TiO2 interface, we focus on the C-C inter ring vibrational mode of N719 at 1307 cm-1 whose intensity is sensitive 14


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to the interfacial charge transfer process depending on the electrode potential. In our previous work16-17, bell-shaped potential-dependency of this band intensity has been observed on Ag@TiO2 and Ag2@TiO2 core-shell particles with the potentials of maximum intensity at -0.7 V with 638 nm and -0.3 V with 785 nm laser excitations. In the present work, we investigate PICT by comparing the potential-dependency of relative intensity of the band at 1307 cm-1 on TiO2 P25 and rutile(110) surfaces at a fixed laser excitation line of 638 nm. It is found that on P25 film electrode, the band intensity gradually increases from 0 V, reaching a maximum at about -0.7 V, and then decreases at more negative potentials. In this case, the potential for reaching Raman intensity maximum is the same as that on Ag@TiO2 NPs. We mention that the 2 nm TiO2 shell of the Ag@TiO2 NPs adopts the anatase phase similar to P25 nanoporous film after annealing at 192 ℃ and exhibits similar photo-electrochemical behavior. Interestingly, on the TiO2 rutile(110) surface, the potential of intensity maximum locates at -0.45 V as shown in Figure 6A.

Figure 5. Schematic illustrations for configurations of N719 adsorbed onto (A) rutile(110) and (B) P25 TiO2 surfaces. Both of the two SCN groups on a N719 are used to adsorb on two P25 nanoparticle surfaces while only one of the two SCN groups is used for adsorption of the molecule on rutile(110) 15



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surface.

Noteworthy is that for a semiconductor electrode the photo-induced charge transfer occurs through the conduction band of the electrode while the control of electrode potential is through the adjustment of Fermi level of the electrode. However, the conduction band edge in the bulk region of a semiconductor electrode is moved following the same trend as Fermi level adjustment, maintaining the energy difference between conduction band edge and Fermi level fixed. Rutile(110) and anatase-dominated P25 of TiO2 belong to the same type of semiconductor but with different crystallographic structures and thus different conduction band edges. The conduction band edge of the rutile is reported to be 0.2 eV higher than that of anatase as revealed by X-ray photoemission measurement and QM/MM calculation.37 However, under a specified laser excitation, for PICT resonance to occur at electrochemically controlled TiO2-N719 interfaces, the conduction band edges of rutile(110) and P25 electrodes should be brought to the same energy position from the consideration of energy level alignment between TiO2 and the N719 (via HOMO), meaning that the potentials of Raman intensity maximum for the two electrodes must be different. Given a specific type of TiO2 electrode, the exact potential at energy level alignment depends on the energy difference of the Fermi level from the conduction band edge of the electrode. Flat band potential is a characteristic potential of a semiconductor electrode at which the electrode carries no excess charge and thus causes no band bending. It is often used to measure the Fermi level of a semiconductor with no band bending. Here we inspect the flat band potential difference between rutile(110) and P25 TiO2 electrodes to verify the potential difference for maximum Raman intensity. Electrochemical impedance spectroscopy is therefore carried out in acetonitrile solution containing 10 mM LiClO4 as supporting electrolyte. Mott-Schottky plot is extracted from the frequency response analysis (FRA) potential scan measurements. As shown in Figure 6B, linear plots are obtained for both the rutile (110) and P25 nanoporous electrodes, and the flat band potential Efb is measured to be 16


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-1.13 V for the rutile (110) and -1.37 V for the P25 nanoporous electrodes, about 0.24 V difference in potential, or equivalent of -0.24 eV difference in Femi level, rutile (110) vs. P25.

The measured flat band potentials are in agreement with those reported in the

work of Kraus el al38 in which the flat band potentials of -1.0 V and -1.2 V (vs. Ag/AgCl) were measured for rutile(110) and anatase (101), respectively, in an aqueous solution using 0.1 M NaClO4 as the electrolyte, i.e. the flat band potential of rutile(110) is about 0.2 V positive of P25. The facts that the conduction band edge of rutile(110) is 0.2 eV higher than that of anatase TiO2 according to ref. 38 while the Femi level of rutile(110) is 0.24 eV lower than that of P25 reveals that the TiO2 rutile (110) has a wider gap between Fermi level and

conduction band edge , about 0.24 eV, than P25 TiO2. It has

been reported39 that the carrier density of sol-gel synthetized TiO2 nanoparticles like P25 are about 1.91019 cm-3 while rutile(110) single crystal are about 51017 cm-3. These evidence are in support of our observed difference in Fermi level against the conduction band edge for the rutile(110) and P25 TiO2 electrodes. And this explains the observed potential difference of 0.25 V, rutile(110) vs. P25, for Raman intensity maximum of the C-C inter ring vibrational mode at PICT resonance.

Figure 6. (a) Potential dependency of relative intensity of the C–C inter ring mode at 1307 cm-1 of N719 adsorbed on P25 (red) and rutile(110) (black) of TiO2, (b) Mott-Schottky plot of rutile(110) and P25 in acetonitrile solution containing 10 mM LiClO4.

17



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CONCLUSION We have demonstrated the method to study single crystalline semiconductor-molecule interfaces using

SHINERS under electrochemical control. Adsorption of N719

molecules onto rutile (110) and P25 nanoporous surfaces and the charge transfer process across the interfaces are compared to elucidate the intrinsic nature of N719 binding to TiO2 surfaces. It has been revealed that the binding of SCN group to the TiO2 surface is the dominant interaction between N719 and TiO2 surface after removing the possible bonding complexity by surface roughness by using single crystalline rutile (110). Both of the SCN groups of N719 are used for binding to P25 while only one used to bind to rutile(110). Furthermore, hydrogen bonding is more favored on atomically flat rutile (110) than on nanoporous P25 surfaces as revealed by the difference in

Raman shift of the

(C=O) (COOH) band. The EC-SERS results have shown that PICT occurs on both P25 and rutile(110) TiO2 surfaces and the potential to achieve PICT resonance depends on not only the excitation energy of light, but also the band structure of the semiconductor. Our work has demonstrated that EC-SHINERS can be successfully applied to single crystalline semiconductor electrodes for fundamental researches, and further applications of this technique may be extended towards photovoltaic and photocatalytic systems for thorough understanding of the interactions at the interfaces. ASSOCIATED CONTENT 18


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Supporting Information.

AUTHOR INFORMATION Measured optical parameters of the rutile(110) single crystalline electrode after Argon sputtering-annealing process by spectroscopic ellipsometry,

3D

Schematic

illustration

of

the

adsorption

configuration

of

rutile(110)/N719 interface, photo-electrochemical response of N719 sensitized rutile (110) and P25 nanoporous electrode. This information is available free of charge via the Internet at http://pubs.acs.org/.

Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors acknowledge the supports of MOST (Grant 2012CB932902) and NSFC (21473147, 21321062). The authors are grateful to Profs. De-Yin Wu at Xiamen University for valuable discussion and assistance.

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Table of Contents Graphic

23


Adsorption of Dye Molecules on Single Crystalline Semiconductor

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