TiO2 Interface by Surface

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Investigation of Charge Transfer in Ag/N719/TiO2 Interface by Surface-Enhanced Raman Spectroscopy Xiaolei Wang, Yue Wang, Huimin Sui, Xiaolei Zhang, Hongyang Su, Weina Cheng, Xiao Xia Han, and Bing Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03228 • Publication Date (Web): 01 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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The Journal of Physical Chemistry

Investigation of Charge Transfer in Ag/N719/TiO2 Interface by Surface-Enhanced Raman Spectroscopy Xiaolei Wang, Yue Wang, Huimin Sui, Xiaolei Zhang, Hongyang Su, Weina Cheng, Xiao Xia Han* and Bing Zhao* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China *To whom correspondence should be addressed. E-mail: [email protected] (XXH); [email protected] (BZ),Tel: +86-431-85168473,Fax: +86-431-85193421.

Abstract The interfaces of metal−dye molecule-semiconductor sandwich structure are very important in the investigation of dye-sensitized solar cells (DSSCs) where metals are used to enhance absorption. In this work, we firstly designed and synthesized Ag/N719 and Ag/N719/TiO2 sandwich systems to investigate chemical binding type at the interfaces of Ag/N719/TiO2. The results of the Raman spectra under the laser excitations of 532, 633, and 785 nm clarified that the SCN groups adsorbed on Ag surface via the S terminal and the TiO2 layer possibly bound to Ag/N719 via the ester linkage (-O-C=O) of the COOH group in N719. Then, we optimized the Ag substrate as SERS detection platform and selected the Ag sol film as the substrate. At last, the relationship between the “degree of CT (ρCT)” in the SERS spectra and the charge transfer (CT) process was investigated by tuning the contribution from chemical effect. We found that owing to the introduction of TiO2, the intensity and ρCT firstly increased (n=0-2) and then decreased (n=2-3) with the increase of the number of TiO2 layers under 532 and 633 nm laser excitations. However, the intensity decreased (n=0-3) with the increase of the number of TiO2 layers under the laser excitation of 785 nm, and there was no obvious change about ρCT. Meanwhile ρCT became higher with the increase of the laser excitation energy at the interfaces with the same TiO2 layer number. In order to explain these variation about ρCT, we utilize UPS and UV-Vis spectra to calculate energy level for better understanding the charge transfer (CT) process, and the calculation result is in accordance 1

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with the variation tendency of ρCT .

Introduction Recently, more and more attention has been paid to dye-sensitized solar cells (DSSCs) due to their low-cost alternatives to conventional solid-state photovoltaic devices.1-3 The most successfully employed charge-transfer sensitizers in DSSCs are Ru(II)-polypyridyl dyes,4,5 and the well known dye (Bu4N)2[Ru(dcbpyH)2-(NCS)2] (N719) attracted increasing attention.6 The dye absorbs light to generate electron, then the photoexcited electron transfers to the conduction band (CB) of the TiO2, which is bound via the carboxylate groups of the dye.7 The functional groups of the dyes allow efficient anchoring on semiconductor surface, which can promote the electronic communication between the donor orbital of the dye and the conduction band of the semiconductors. To our knowledge, there are two kinds of anchor groups (SCN and COOH) of N719. As reported, the N719 connected with metals by the SCN groups via S or N terminal,8,9 and semiconductors adsorb on monolayer of N719 via COOH in various types.10 To study interfacial interactions, which govern the adsorption configuration of molecules and affect the efficiency of charge transfer (CT) process at the interface, vibrational spectroscopy, such as infrared (IR)11,12 and Raman spectroscopies,13,14 serve as powerful tools. Nowadays, surface-enhanced Raman scattering (SERS) technique has attracted more and more attentions15-18 due to its properties of high sensitivity and quenching of fluorescence.19-22 SERS spectroscopy can provide highly sensitive fingerprint information to realize the detection of monolayer dye film in DSSCs. As widely accepted that SERS is a complex phenomenon originating from two main factors,23,24 one is the electromagnetic mechanism (EM), which is due to resonance of the incident

2


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electromagnetic field with the surface plasmon of the metallic nanostructure.25 The other is the chemical mechanism (CM), which is due to photoinduced charge transfer (CT) between the metal (or semiconductor) and the adsorbed molecules. In DSSCs, the efficiency of charge transfer (CT) is essential, which is the working principle of solar cells.26 Up to now, there are many researchers that utilize SERS technology to investigate N719 adsorbed on noble metals e.g., Ag, Au and Cu surfaces. León et al.27 completed the specific peak assignments of N719 adsorbed on Ag and Au NPs based on previous report for RS,28 RRS,29 SERS,30,31 and SERRS.32 Joo33 presented that N719 had a relatively perpendicular geometry with its bipyridine ring on the metal surfaces and the carboxyl groups would likely be deprotonated. The occurrence of charge transfer between N719 and metal electrode surfaces had been examined by the variation of ν(SCN) in SERS spectra. Theil et al.34 identified the binding site of N719 onto the metal surface by Raman, RR, and SERRS studies. Some minor changes in the relative intensity patterns of the Raman bands such as 1476 and 1541 cm−1 in the SERRS spectra of N719 as compared to its RR spectra were observed and explained in terms of the binding geometry of the dyes on the surface. In order to study size-dependent effect in fundamental SERS studies, Schade et al.35 carried out SERS measurements of N719 on nanoporous gold fabricated by the photothermal laser processing approach, whose pore size varies from a few tens of nanometer to several microns. The results exhibited a strong size-dependence of the Raman signals of N719, and the highest intensities were observed on the gold material with average pore sizes of about 25 nm. Now, noble metal nanoparticles were employed to construct the new kind of DSSCs, where design approaches based on plasmon can be used to improve absorption in DSSCs.

36-38

Tian’s

group39,40 have employed Ag@TiO2 and Ag2@TiO2 core–shell nanoparticles to investigate the 3



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interface of TiO2–N719 dye by situ electrochemical SERS (EC-SERS) with systematical potential control. The potential dependence of EC-SERS discloses the binding method at the interface of TiO2–N719. Furthermore, the impact of various excitation wavelengths (638 and 785 nm) on the potential dependence of EC-SERS verifies the photon-driven charge transfer (PICT) mechanism from the HOMO of N719 to the CB of TiO2. In our work, we utilized the more controllable and ordered TiO2 layer instead of this kind of Ag@TiO2 core-shell NPs to investigate the CT process in the Ag-dye-TiO2 sandwich structure based DSSCs with Ag. The change of contact interface may change the charge energy level at the interface, resulting in the different CT process. With the introduction of TiO2 layers, the contribution from CM effect has changed. For estimating the specific CM contribution to the whole SERS intensity quantitatively, we used the conception of the “degree of CT (ρCT)”, proposed by Lombardi et al.41,42 Thus, we not only tuned the CM effect, but also described it quantitatively by ρCT for better understanding the various CT process under different conditions. These may provide new thinking that ρCT in the SERS spectra which has been well understood can be used to describe the CT process in DSSCs and used as qualitative evaluation method for the performance of it, such as monochromatic incident photon-to-electron conversion efficiency (IPCE), and for further optimizing the performance of the sensitized cells and enhance their quantum yield.43,44 In this work, we firstly explored a specific chemical anchoring form among the Ag, N719 and TiO2. Then, we optimized the Ag substrate as SERS detection platform and selected the Ag sol film as the substrate in the following experiment. Furthermore, the CM effect for SERS simultaneously was tuned in the Ag/N719/n-TiO2 structure for better understanding the various CT process under different conditions, which was evaluated by the conception of the “degree of CT (ρCT)” 4


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qualitatively. We assembled various number TiO2 layers on Ag/N719 substrates controllably and orderly to tune the CM effect in Ag/N719/n-TiO2 system. Finally, we get the energy level scheme of the sandwich structure to investigate the CT process in these system.

Experimental section Chemicals. Poly(diallyldimethylammonium chloride) (PDDA, Mw = 200 000-350 000, 20 wt % aqueous solution) and (Bu4N)2[Ru(dcbpyH)2-(NCS)2] (Bu4N = tetrabutylammonium and dcbpy = dicarboxylbipyridine) N719 were purchased from Sigma. All other chemicals were analytic grade and were acquired from Beijing Chemical Reagent Factory and used without further purification.

Preparation of Silver NPs. In this work, we utilized two methods to prepare different Ag films as substrates, self assembly and vacuum deposition method. (a) Self-assembly Ag sol film. The silver colloid was prepared using a conventional synthetic route that has been reported elsewhere.45 Briefly, 200 mL of a 1.0 mM aqueous silver nitrate solution was added to 250 mL three-neck bottles and then heated to 85 °C with rapid stirring under reflux. A 4 mL solution of 1% trisodium citrate was added to the solution, after which it was boiled for 40 min. Silicon wafers were immersed in a boiling solution prepared by mixing 30% H2O2 and 98% H2SO4 with a volume ratio of 3:7. After cooling, the silicon wafers were rinsed repeatedly with water. As a result, the silicon surface was covered with hydroxyl groups. Then, they were immersed in a 0.5% PDDA solution for 1h. After exhaustive rinsing by water and drying by nitrogen, the PDDA coated slides were soaked in silver colloid for 6 h and then rinsed with water. (b) Ag island film prepared by vacuum deposition. Silicon wafers were coated with a thermally vapor-deposited, densely packed film of 5, 10, 15 and 100 nm Ag island films separately. The Ag island films were thermally vapor-deposited in a vacuum chamber at the vacuum degree of about 5



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1.33×10-3 Pa. The deposition rate was kept at 0.05 nm/s throughout the deposition process.

Preparation of Ag/N719. Silicon wafers assembled with Ag NPs were immersed in N719 solution (10-6 M) for 12 h at room temperature. The samples were removed from the solution and rinsed with distilled water for five times.

Preparation of TiO2 Layers. The Ag/N719/n-TiO2 sandwich systems were assembled as the surface sol gel (SSG) method originally developed by Ichinose et al.46,47 The formation of the Ag/N719/n-TiO2 sandwich system can be distinctly described by Figure 1a. The silicon wafers assembled with Ag/N719 were transferred into tetrabutyl titanate (100 mM in a 1:1 (v/v) mixture of toluene/methanol) for 1h.

Then the silicon wafers were washed with methanol and dried with

nitrogen gas forming the structure as shown in Figure 1a(2). There is a transesterification between COOH and tetrabutyl titanate in this step, where double COOH groups losing H atoms and single tetrabutyl titanate losing two tetrabutyl groups. Finally, the silicon wafers were submerged in water for 1min. Then the silicon wafers were washed with methanol and dried with nitrogen gas. Thus the monolayer of TiO2 formed as shown in Figure1a(3) and one adsorption cycle was completed. Additional TiO2 layers were fabricated by repeating the cycle to prepare the Ag/N719/n-TiO2 sandwich system.

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Figure. 1 Illustration of the stepwise assembly of the Ag /N719/n-TiO2 sandwich system (a), and molecular structure of N719 (b).

Instrument. The UV-Vis-NIR spectra were recorded on a Shimadzu UV-3600 spectrophotometer. TEM images were taken using a JEOL JSM-6700F field-emission scanning electron microscope (FE-SEM) operating at 3.0 kV. SERS spectra were measured on a Horiba Jobin Yvon T64000 and Renishaw1000 system with excitation wavelengths of 532, 633, and 785 nm. The laser beam was focused onto a spot whose diameter is approximately 1μm using an objective microscope with a magnification of 50×. The data acquisition time was 60 s. The Raman band of the silicon wafer at 520.7 cm-1 was used to calibrate the spectrometer. Due to the restriction of the instrument, the wavenumber region for measurements is 200-2000cm-1 under 785 nm laser excitation, and it is 200-2300cm-1 under the laser excitations of 532 and 633 nm.

Results and Discussion Formation and characterization of Ag/N719 and Ag/N719/TiO2. UV−Vis Spectra. The optical properties of different systems were investigated employing UV-Vis spectrometry. We assembled Ag, Ag/N719, and Ag/N719/TiO2 systems on glass slides and obtained the UV-Vis absorbance spectra as shown in Figure 2. The surface plasmon resonance (SPR) absorption peak of Ag sol film located at 419 nm for the particles assembled on the glass slide (Figure 2a). After the adsorption of N719 molecules on the Ag sol film (Ag N719, Figure 2b), there is no obvious change as compared with Ag sol film (Figure 2a). After the introduction of TiO2, the SPR absorption peak shows an obvious red-shift from 419 nm (Ag/N719, Figure 2b) to 428 nm (Ag/N719/TiO2, Figure 2c) , which is caused by the larger refractive index of TiO2 covered the Ag sol film.48 Meanwhile , a new absorption band appears in the 460-550 nm regions. According to Mie scattering theory, if the refractive index of the surrounding medium is changed, the change of SPR 7



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absorption band shows only the shift of peak position without the change of peak width or intensity.49 Thus, it can be inferred that this new absorption band results from the formation of a new surface energy state in the CT process of Ag/N719/TiO2 system. And this hypothesis will be demonstrated by SERS in the future discussion of this work.50 The changes of the absorbance peaks in UV-Vis spectra of Ag, Ag/N719 and Ag/N719/TiO2 systems showed that the Ag/N719/TiO2 system was constructed successfully.

Figure 2. UV-vis absorption spectra of Ag (a), Ag /N719 (b) , and Ag/N719/TiO2 (c) .

SERS Spectra. In order to investigate the structural information about the chemisorption on the surfaces of Ag/N719 system, we compare the Raman spectra of N719 and Ag/N719 as shown in Figure 3. The assignment of the main bands of N719 has been described previously in the literature.27 For convenience, Table S1, which summarizes the band assignments, is provided as Supporting Information (SI). It is noted that when N719 was absorbed on Ag sol film, the band of N719 at 2096 cm-1 associated with ν(C=N) of isothiocyanate group clearly shifts to 2150 cm-1 8


51-53

and the relative

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intensity of ν(C=N)(SCN) compared to the modes of the polypyridine ligands at 1469 cm-1 turned to be stronger. As reported in previous work,34 the isothiocyanate vibration is barely enhanced by the resonance effect. These spectral changes about the isothiocyanate ligands between the normal Raman (Figure 3a) and SERS(Figure 3b) spectra can be explained by a strong SERS enhancement because of the chemical adsorption of the isothiocyanate ligand to the metal surface. In low wavenumber region of the Ag/N719, a new band appears at 242cm-1 due to ν(Ag-S). Thus, it is clear that SCN group absorbs on Ag sol film via the S terminal, which is in accordance with the previous study.54 The blue-shifts of δ(SCN) bands from 450 (Figure 3a) to 458 cm-1 (Figure 3b, c, d) means the changes of spatial structure in geometry.

Figure 3. Raman spectrum of N719 powders with 532 nm excitation (a), and SERS spectra of Ag/N719 at the excitations of 532 nm (b), 633 nm (c) , and (d) 785 nm. As compared to the normal Raman spectrum (Figure 3a), in the SERS spectra (Figure 3b, c, d) 9



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we can observe the appearance of the asymmetric and symmetric ν(COO-) stretching bands at 1558 and 1370 cm-1 respectively, which is especially significant under 633 and 785 nm excitation line.27 The intensity of ν(C=O) at 1722 cm−1 decreases in the Ag/N719 system (Figure 3b, c, d). However, it is too weak to indentified under 633 and 785nm excitation line, which indicate that the COONBu 4 groups of N719 should be deprotonated and the COOH groups still exist in the Ag/N719 system. In the bipyridine predominated Raman spectrum of N719 powder, the vibrational bands at 1609, 1540, 1469, 1427,1300, 1266, and 1019 cm-1 can be ascribed to the stretching modes of C=C and C=N of the bipyridine rings, whose intensity is enhanced due to resonance effect. The vibrations of the polypyridine ligands also reveal some differences between the SERS and the normal Raman spectra. The intensity of the bands at 1474 and 1540 cm−1 in the SERS spectra of Ag/N719 increases in different degree compared to the Raman spectrum of N719. The band at 1474 cm−1 can be assigned to the stretching mode of C=C that are connected with the carboxylate group in the bipyridine. N719 is expected to have a perpendicular orientation on the surface of Ag sol film binding via the S terminal in SCN group. As the SERS dipole selection rules predict that the vibrational mode perpendicular to the metal surface has a higher SERS enhancement for as compared to the parallel one,55,56 the mode at 1474 cm−1 has a higher vibrational component perpendicular to the metal surface than the mode at 1540 cm−1 which consists of ring deformation parallel to the metal surface. This assumption is in accordance with the DFT calculations for the assignments of these two above Raman bands. 34 The selective enhancement of the band at 1167 cm-1 in the SERS spectrum of N719 due to the δ(CCH) in-plane deformation can further verify a perpendicular orientation of N719 on Ag sol film.53 The comparison of the SERS spectra under different excitation wavelengths of N719 on Ag sol 10


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film showed that the intensity profile observed was quite different. These differences are expected for a resonant excitation involving specific electronic states of a molecule because the electronically resonant excitation only leads to an enhancement of such vibrations that are coupled to the electronic transition.57 The C=C and C=N stretching bands of bipyridine (1427 cm -1, 1474 cm-1, 1540 cm-1, 1609 cm-1 ) , the bipyridine ring breathing stretching band (1024 cm-1), and the ν(C=O) (1719 cm-1) are enhanced owing to the resonance effect. The νas(COO-) at 1558 cm-1, νs(COO-) at 1370 cm-1, and ν(C=N) from SCN at 2150cm-1 are not enhanced due to the resonance effect. Figure 4 showed SERS spectra of Ag/N719 and Ag/N719/TiO2 systems under the excitation wavelengths of 532, 633, and 785 nm. In Figure 4, the νs(COO-) mode at 1370 cm−1 (Ag/N719) shifted to 1365 cm−1 (Ag/N719/TiO2). The νas(COO-) mode at 1558 cm−1 (Ag/N719) shifted to 1553 cm−1 (Ag/N719/TiO2), which was obvious under 633 and 785 nm excitation. And the ν(C=O) is still

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Figure 4. SERS spectra of Ag/N719 (black line), and Ag/N719/TiO2 (red line) under the excitation wavelengths of 532, 633, and 785 nm. existent which is obvious under 532nm excitation. The existence of νs(COO-), νas(COO-) and ν(C=O) indicated that COO- and COOH still exist in the Ag/N719/TiO2 system as same as in the Ag/N719 system. The TiO2 layer most possibly bound to Ag/N719 via the ester linkage(-O-C=O) of the COOH groups from N719, which is also demonstrated by previous works.10,58-60 The specific discussion of the linkage method are shown in Supporting Information (SI). The ν(C=N) from SCN at 2150 cm−1 (Ag/N719) shifted to 2144 cm−1 (Ag/N719/TiO2). This minor red-shift is due to the whole structure changing in the Ag/N719/TiO2 system rather than the SCN groups directly interact with TiO2.

Optimizing Ag substrate as SERS detection platform. In this work, we prepared Ag sol and Ag island films to fabricate the Ag/N719/TiO2 system. The Ag sol film was prepared by self-assembly method and the Ag island films were prepared by vacuum deposition with different thickness of Ag nanoparticles 5 nm, 10 nm, 15 nm, and 100 nm, respectively. The morphology of these five different Ag substrates were characterized by SEM in Figure S1. Figure S1(a) shows that, on the Ag sol film, the average diameter of Ag NPs is about 60 nm and the density is the smallest among the five Ag substrates. Figure S1(b-e) show that, on the Ag island films, the density of Ag NPs on the wafers obviously increased with the increase of the thickness of Ag island film. It is well known that EM is closely related to the size, shape, and density of metal particles.61-68 Figure 5 (b-e) show the Raman spectra of Ag/N719/TiO2 assembled on Ag island film. The intensity of Raman signal firstly increases (5-10 nm) and then decreases (10-100 nm) with the increase of the thickness of Ag island film. For the Ag island films with thickness of 5 and 10 nm, the larger density and smaller interspace lead to much stronger plasmonic coupling between Ag NPs, resulting in the 12


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increase of Raman intensity. When the thickness of Ag island film exceeds 10 nm in our experiments, the deposited Ag NPs tends to form a continuous film, resulting in the decreasing Raman intensity, which is consistent with previous theoretical and experimental findings. 69 As shown in Figure 5, the changes of SERS intensities from the Ag island films are not notable. In contrast, the Raman intensity of N719/TiO2 assembled on Ag sol film is much higher than those from the Ag island films. Thus, we selected the Ag sol film as the substrate in the following experiments.

Figure 5. Raman spectra of Ag/N719/TiO2 assembled on five different kinds of Ag films with 532 nm excitation. Ag sol film prepared by self assembly method (a) and Ag island film prepared by vacuum deposition with different thickness of Ag nanoparticles 5 nm (b), 10 nm (c), 15 nm (d), and 100 nm (e).

Tuning chemical enhancement effect and the CT Mechanism. In the Ag/N719/n-TiO2 (n=0-3) systems, CM effect was tuned effectively in order to understand the CT process more accurately. In this work, we tuned the CM effect by changing the number of TiO2 layers in the Ag/N719/n-TiO2 system. As the Ag substrate keeps the same, the EM contribution is held fixed. The “degree of CT (ρCT(k))” of a k-bond can be determined using the following equation: 13



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Herein, k is an index used to identify individual molecular lines in the Raman spectrum. We choose the intensities of two lines in a spectral region for better understanding. One line only originates from SPR, whose intensity is denoted as I0(SPR), and for this line Ik(SPR) = I0(SPR). While the other line originates from CT excluding the contribution of SPR, whose intensity is denoted as Ik(CT). In this case, Ik(SPR) is normally very small or zero. One line which we selected to calculate ρCT is the ring breathing mode at 1024 cm−1, which is enhanced with the whole spectrum. And the other line is the C-C intern-ring C=N stretching modes of N719 at 1266 cm-1, which is strongly affected by the charge transfer (CT) process caused by adsorption and SERS effects, as reported by Leon’s group.10 And this band is enhanced selectively changing the shape of spectrum. Thus the equation (1) can be approximately expressed as follows:

Thus, the value of ρCT is proportional to the contribution from CM enhancement. We tuned the CM effect, and described it quantitatively by ρCT furthermore for better understanding the various CT process under different conditions. In this way, the change of CT process caused by the various CM effect can be evaluated by “degree of CT (ρCT)” qualitatively. Tuning chemical enhancement effect and the CT Mechanism. Figure 6 (A, B, and C) show SERS spectra of Ag/N719/n-TiO2 (n=0-3) under the laser excitation wavelengths of 532, 633, and 785 nm, respectively. Figure 6 (D) shows a plot of ρCT as a function of excitation wavelengths. After layer by layer adsorption of TiO2, the intensity and ρCT firstly increases (n=0-2) and then decreases (n=2-3) 14


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with the increase of the number of TiO2 layers under 532 and 633 nm laser excitations. ρCT increases from 0.466 (n=0) to 0.563 (n=2) under 532 nm laser excitation, and ρCT increases from 0.414 (n=0) to 0.502 (n=2) under 633 nm laser excitation. But under 785 nm laser excitation, the intensity decreases (n=0-3) with the increase of the number of TiO2 layers, and there is no obvious change about ρCT. Meanwhile ρCT becomes higher with the increasing laser excitation energy on the TiO2 with the same layer number. In order to explain these variation about ρCT, we utilize the UPS and UV-vis spectra to calculate energy level. As the calculation results shown in Figure S2-Figure S8, the Fermi level of Ag is situated at 4.84 eV from the vacuum level. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) level of N719 are at 6.73 and 4.26 eV, respectively. The HOMO and LUMO level of various number of layers TiO2 are situated at 5.62 and 8.52 eV (n=1), 5.68 and 8.52 eV (n=2), and 5.90 and 8.59 eV (n=3).

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Figure 6. SERS spectra of Ag/N719/0-TiO2 (red line), Ag/N719/1-TiO2 (blue line), Ag/N719/2-TiO2 (purple line) , and Ag/N719/3-TiO2 (pink line) under the excitation wavelength of 532 nm (a), 633 nm (b), and 785nm (c). the degree of CT for N719 as a function of the layer number (n) of TiO2 for three different excitation wavelengths (532, 633, and 785 nm) is shown in (d).

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Figure 7. Illustration of CT mechanism of N719 in (a) Ag/N719 and (b) Ag/N719/n-TiO2 nanosystem.

In metal-organic molecule system, there will be charge redistribution around the interface within a short time forming a new electronic state. 70 In the system of Ag/N719, N719 absorbs different energy of the incident lasers to generate different charge transfer processes, which lead to the differences in SERS signals. As shown in Figure7(a), there are three parts of contributions to SERS: (1) the contribution from the electromagnetic enhancement of Ag, whose intensity we denoted as IEM. (2) the contribution from the resonance effect, in which the photo-excited electron resonantly transfers from the HOMO level of N719 (6.73 eV) to the LUMO level of N719 (4.26 eV). And we denoted the intensity as IRR. (3) The contribution from the CT process between the HOMO level of N719 (6.73 eV) and the Fermi level of Ag (4.84 eV), whose intensity we denoted as ICT(Ag-N719). Thus, in the system of Ag/N719, the intensity of SERS can be expressed as follows:

The contribution of CT process and the resonance effect do not exist all the time due to the limit of laser wavelengths. The specific contribution to SERS under the excitation wavelength of 532 nm, 17



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633 nm, and 785nm are shown in Table 1. Thus, in the Ag/N719 system, the SERS intensity and ρCT increase with the increase of the laser excitation energy, which is in accordance with the variation tendency of the SERS results in our study. Table 1. The contribution to SERS under the excitation wavelength of 532 nm, 633 nm, and 785nm. The laser excitation

The laser excitation

wavelength/ nm

energy/ eV

532

2.33

633

785

The contribution to SERS Ag/N719

1.96

1.58

Ag/N719/TiO2

EM, RR,

EM, RR,

CT(Ag-N719)

CT(Ag-N719), CT(N719-TiO2)

EM,

EM,

CT(Ag-N719)

CT(Ag-N719), CT(N719-TiO2)

EM

EM, CT(N719-TiO2)

After the introduction of various number of TiO2 layers to the Ag/N719 system, charge redistribution occurs around the interface between the Ag, N719 and TiO 2 layers. As shown in Figure7(b), the above three parts contribution to SERS are still exist in this system. Additionally, a new part of contribution appears: the contribution from the CT process between the HOMO level of N719 (6.73 eV) and the LUMO level of TiO2 (5.32 eV), which we denoted as ICT(N719-TiO2). Thus, in the system of Ag/N719/TiO2, the intensity of SERS can be expressed as follows:

As shown in Table 1, the additional contribution from CT(N719-TiO2) leaded to the higher SERS intensity and ρCT in the Ag/N719/n-TiO2 system as compared with those in the Ag/N719 system. 18


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Moreover, with the increase of the number of TiO2 layers, the TiO2 layer further induced the charge transfer accompanied with the increase of ρCT. Thus, ρCT reached the highest under the laser excitation of 532 and 633 nm when the number of TiO2 layers is 2. However, if the number of TiO2 layers further increased (>2), Raman scattering of N719 would be affected by the TiO2 colloid layers (thicker TiO2 layers would be barriers for light scattering), resulting in the decrease of the ρCT in the Ag/N719/3-TiO2 system. Because of the relative small energy for the laser of 785 nm, the contribution of CT(N719-TiO2) was not obvious in the Ag/N719/n-TiO2 system, and thus, there was no obvious change about the SERS intensity and ρCT.

Conclusions In conclusion, we firstly designed and synthesized Ag/N719 and Ag/N719/TiO2 sandwich systems to investigate chemical binding at the Ag/N719/TiO2 interfaces. The laser-dependent Raman spectra disclosed that the SCN adsorbs on Ag NPs surface via the S terminal and TiO2 layer most possibly bound to Ag/N719 via the ester linkage(-O-C=O) of COOH from N719. Then, we optimized the Ag substrate as SERS detection platform and selected the Ag sol film as the substrate. At last, we tuned the contribution from the CM effect was tuned by changing the number of TiO2 layers. According to the results, after the introduction of TiO2, the intensity and ρCT firstly increases (n=0-2) and then decreases (n=2-3) with the increasing the increase of the number of TiO2 layers under 532 and 633 nm laser excitations. However, the intensity decreases (n=0-3) with the increase of the number of TiO2 layers, and there is no obvious change about ρCT under the laser excitation of 785 nm. In order to explain these variation about ρCT, we utilize the UPS and UV-Vis spectra to calculate energy level for better understanding the charge transfer (CT) process, whose calculation result is in accordance 19



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with the variation tendency of ρCT. The interfaces of metal−dye molecule-semiconductor sandwich structure are very important in the investigation of dye-sensitized solar cells (DSSCs) where metals are used to enhance absorption. This study not only clarified the specific chemical binding mode of N719 at the Ag/N719/TiO2 Interface but also provided a relationship between the “degree of CT (ρCT)” in the SERS spectra and the charge transfer (CT) process, which is beneficial for better understanding of DSSCs.

Supporting Information Assignments of major bands for N719, Ag/N719, and Ag/N719/TiO2; The discussion of the binding type of COO-; SEM images of different Ag substrates; The calculation of energy level: UPS of Ag, Ag/N719 and Ag/N719/n-TiO2 system, UV-vis spectra of N719, and Ag /N719/n-TiO2 system. The Raman intensity dependency on pump power. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgements The Research was Supported by the National Natural Science Foundation (Grant Nos. 21273091, 21221063, 21327803, 21411140235, 21403082) of P. R. China.

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Figure. 1 Illustration of the stepwise assembly of the Ag /N719/n-TiO2 sandwich system (a), and molecular structure of N719 (b).

Figure 2. UV-vis absorption spectra of Ag (a), Ag /N719 (b) , and Ag/N719/TiO2 (c) .

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Figure 3. Raman spectrum of N719 powders with 532 nm excitation (a), and SERS spectra of Ag/N719 at the excitations of 532 nm (b), 633 nm (c) , and (d) 785 nm.

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Figure 4. SERS spectra of Ag/N719 (black line), and Ag/N719/TiO2 (red line) under the excitation wavelengths of 532, 633, and 785 nm.

Figure 5. Raman spectra of Ag/N719/TiO2 assembled on five different kinds of Ag films with 532 nm excitation. Ag sol film prepared by self assembly method (a) and Ag island film prepared by vacuum deposition with different thickness of Ag nanoparticles 5 nm (b), 10 nm (c), 15 nm (d), and 100 nm (e). 28


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Figure 6. SERS spectra of Ag/N719/0-TiO2 (red line), Ag/N719/1-TiO2 (blue line), Ag/N719/2-TiO2 (purple line) , and Ag/N719/3-TiO2 (pink line) under the excitation wavelength of 532 nm (a), 633 nm (b), and 785nm (c). the degree of CT for N719 as a function of the layer number (n) of TiO2 for three different excitation wavelengths (532, 633, and 785 nm) is shown in (d).

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Figure 7. Illustration of CT mechanism of N719 in (a) Ag/N719 and (b) Ag/N719/n-TiO2 nanosystem.

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TiO2 Interface by Surface

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