Ag Interface Monitored by Surface

Article pubs.acs.org/JPCC

Charge Transfer at the TiO2/N3/Ag Interface Monitored by SurfaceEnhanced Raman Spectroscopy Xiaolei Wang,† Bing Zhao,† Peng Li,† Xiao Xia Han,*,† and Yukihiro Ozaki‡ †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan

‡

S Supporting Information *

ABSTRACT: The interface of semiconductor−dye−metal system is a crucial issue for investigating dye-sensitized solar cells (DSSCs), where the electron transfer takes place. In this work, a series of assemblies of TiO2/N3 (cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′dicarboxylato)ruthenium(II)) and TiO2/N3/Ag have been fabricated, which were employed for the investigation of the adsorption configuration and conformational change of N3 molecules. We plot degree of charge transfer (CT) (ρCT) as a function of excitation wavelength of TiO2/N3 and TiO2/N3/Ag assemblies, which contributes to the understanding of the CT process in the series of N3 assemblies. According to the variation tendency of ρCT, when laser energy exceeds the CT energy threshold 2.071 eV, ρCT shows an obvious increasing trend with the increasing laser energy. In the case of TiO2/N3/Ag assembly, when the laser energy exceeds the CT energy threshold 1.877 eV, ρCT becomes lager with the increase in the laser energy, until asymptotic behavior appears under higher laser energy. To explain the variation tendency of ρCT and the shift of CT energy threshold, we have proposed two models about the energy level scheme of TiO2/N3 and TiO2/N3/Ag assemblies. Furthermore, we investigated the influence of crystal structure of TiO2 NPs on the CT process by the fabrication TiO2/N3/Ag assemblies based on anatase and rutile TiO2 NPs. It is noted that the TiO2/N3/Ag assembly based on TiO2 NPs calcinated at 450 °C with highest ρCT and lowest CT energy threshold is most in favor of CT process. Besides the specific chemical binding mode in the TiO2/N3/Ag system, this study also found the relationship between the ρCT and the CT process, which is of considerable importance and relevance to solar energy conversion.

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INTRODUCTION Recently, dye-sensitized solar cells (DSSCs) have received more and more interest because of their low cost, long durability, high efficiency, and potential as alternatives to conventional solid-state photovoltaic devices.1−3The dye anchored on semiconductor surface is an important component of DSSCs, which can promote high quantum yields of the electron transfer process. Thus, the efficiency of the charge transfer (CT) in DSSCs is highly dependent on the anchoring type between the dye and semiconductor. N3 molecules assembled onto the surface of TiO2 have been extensively investigated.4−8 Finnie et al.8 concluded based on an FTIR/Raman study that the COOH groups attached with bidentate or bridge type to the surface of TiO2. Nazeeruddin et al.9 concluded similar results with ATR-FTIR data, and they found N3 anchored to the TiO2 surface via the two out of four carboxyl groups coming from two different bipyridine ligands in a bridging coordination. Furthermore, Johansson et al.10 found by a photoelectron spectroscopy study that one of SCN groups on the dye molecule can also interact with TiO2 surface via a hydrogen bonding through the sulfur atom, and for this binding configuration the fraction is only 30% with most of the SCN groups remaining free. More recently, a new kind of DSSC with the addition of noble metal NPs, which can improve light absorption in DSSCs,11−13 has been constructed. Tian’s group14,15 have © XXXX American Chemical Society

utilized Ag@TiO2 and Ag2@TiO2 core−shell NPs for electrochemical SERS (EC-SERS) investigations of N719 (ditetrabutylammonium cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)ruthenium(II)) adsorbed on TiO2. Variation of EC-SERS disclosed the bond mode in TiO2−N719 system. Meanwhile, it has been proved that the photoinducedCT (PICT) process depends on the excitation energy of light. In this new kind of DSSCs, by taking advantage of the surface plasmonic resonance of the Ag, the intensity of Raman signals is largely enhanced, which allows for preresonance and nonresonance Raman investigations. The electromagnetic mechanism (EM) and the chemical mechanism (CM) are two major accepted mechanisms16−20 for SERS. The former is based on the local surface plasmon resonance (LSPR) generated by the adjacent metal nanostructures, whereas the latter is caused by photoinduced CT between the metal (or semiconductor) and the adsorbed molecules. After introduction of a metal into dye molecule− semiconductor system, apart from the enhancement of EM, the contribution from the CM effect is also enhanced, which can lead to the change of CT process. Thus, it is meaningful to study the contribution of CM separately from the whole SERS Received: January 6, 2017 Revised: February 15, 2017 Published: February 16, 2017 A

DOI: 10.1021/acs.jpcc.7b00153 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Representative TEM images of the TiO2 NPs (A), TiO2/N3 (B), and TiO2/N3/Ag (C) assemblies; SEM image of the Ag NPs assembled on glass slide (D); size distribution of the TiO2 NPs (E) and Ag NPs (F).

Ag based on anatase TiO2 NPs calcinated at 450 °C have been fabricated by a self-assembly method. Apart from the investigation of the adsorption configuration and conformational change of N3 by Raman spectroscopy, we also have estimated the CM contribution to the whole SERS intensity of TiO2/N3 and TiO2/N3/Ag assemblies under the 476.5, 514.5, 532, 633, and 785 nm excitation. In the present study the values of ρCT under various laser excitation wavelengths are compared, and ρCT is also plotted as a function of excitation wavelength of TiO2/N3 and TiO2/N3/Ag, which enables us to obtain CT energy threshold. Furthermore, we have proposed two models about the energy level scheme of TiO2/N3 and TiO2/N3/Ag to explain the variation tendency of ρCT and the shift of CT energy threshold. Meanwhile, we have also prepared TiO2/N3/ Ag assemblies based on anatase and rutile TiO2 NPs calcinated at 400 °C, 450 °C, and 500 °C to investigate the influence of crystal structure of TiO2 NPs on CT process with the help of ρCT.

intensity. In our previous work,21 we have chosen silver (Ag) as the basic substrate to fabricate metal−dye molecule−semiconductor systems, which contribute to generate strong SERS signal. We successfully employed Ag/N719/n-TiO2 (n = 0−3) systems to explore the binding style and the CT process in it. We found that, compared to Ag/N719 system, the SERS spectrum of N719 in the Ag/N719/n-TiO2 (with appropriate number of TiO2 layers) was selectively enhanced by the CM mechanism. Meanwhile, we have applied the concept of the “degree of CT (ρCT)”, proposed by Lombardi et al.,22,23 to estimate the various CM contributions to the whole SERS intensity in Ag/N719/n-TiO2 systems (n = 0−3) with excitation wavelengths of 532, 633, and 785 nm, which provides a better understanding of the CT process evaluated by ρCT. These methods proposed a new idea by using the wellknown ρCT in SERS to measure the CT extent in DSSCs in order to further optimize parameters for efficient CT, which is beneficial to enhance the quantum yield.23,24 Being different from the previous work, in this work, we have chosen TiO2 as the basic substrate to fabricate semiconductor− dye molecule−metal systems, which are more similar to the structures of DSSCs. Moreover, to explore whether the property of selective enhancement is appropriate for other dye molecules or not, in this work, another dye molecule, N3, was selected. A series of assemblies of TiO2/N3 and TiO2/N3/

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EXPERIMENTAL SECTION Chemicals. The N3 was obtained from Sigma, and all other chemicals were of analytic grade and purchased from Beijing Chemical Reagent Factory. Sample Preparation. Preparation of TiO2 NPs. Various crystal phases of TiO2 NPs in this work were obtained by a B


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The Journal of Physical Chemistry C sol−hydrothermal method according to previous papers.25 First, add a mixture containing 5 mL of tetrabutyl titanate and 5 mL of anhydrous ethanol into another mixture containing 20 mL of anhydrous ethanol and 3 mL of 70% nitric acid to get a transparent sol. After stirring for 1 h, the above sol was kept at 160 °C for 6 h. Then dry it and calcine the product at three different temperatures (400, 450, and 500 °C) for 2 h. Preparation of Ag NPs. Ag colloid used in this work was prepared using the method of Lee and Meisel:26 200 mL of aqueous solution of 10−3 M silver nitrate was reduced by 4 mL of solution of 1% trisodium citrate. Preparation of TiO2/N3 and TiO2/N3/Ag. TiO2/N3 and TiO2/N3/Ag assemblies were prepared by the following procedure in sequence: dissolve 5 mg of TiO2 NPs into 2 mL of N3 (0.01 mM) ethanol solution, and stir it for 12 h at room temperature. Then, centrifuge, rinse, and dry the mixture with anhydrous ethanol to get the precipitation. To get TiO2/ N3/Ag assembly, add the mixture of 1 mL of Ag sol and 1 mL of anhydrous ethanol in as-prepared TiO2/N3 and stir it for 6 h. At last, centrifuge, rinse, and dry the mixture with anhydrous ethanol to get the precipitation. So the TiO2/N3 and TiO2/ N3/Ag assemblies can be prepared. Instruments. X-ray diffraction (XRD) patterns were recorded on a Siemens D5005 X-ray powder diffractometer with a Cu Kα radiation source at 40 kV and 30 mA. Scanning electron microscope (SEM) images were recorded by JEOL JSM-6700F operating at 3.0 kV. Transmission electron microscope (TEM) images were recorded by JEM-2100 at 200 kV. The UV−vis diffuse reflectance spectra (UV−vis DRS) were obtained with a Shimadzu U-4100 UV−vis spectrophotometer. The UV−vis−NIR spectra were obtained with a Shimadzu UV-3600 spectrophotometer. Raman spectra were obtained with Horiba Jobin Yvon T64000 (476.5 and 514.5 nm), Renishaw 1000 (532 nm), and Horiba Jobin Yvon LabRAM ARAMIS system (633 and 785 nm) with various excitation wavelengths. In this work, the Raman spectra of N3 molecule in different assemblies were collected from the solid powder samples.

Figure 2. UV−vis DRS of the TiO2 NPs (A), TiO2/N3 (B), and TiO2/N3/Ag (C) assemblies.

absorption of TiO2/N3 associated with TiO2 band−band transition (below 410 nm) shows a slight red-shift, accompanied by the enhancement of optical absorption (in the 410− 490 nm region) due to TiO2 surface state. These changes are caused by the interaction between the N3 molecule and the TiO2 NPs. A new band appears at 493 nm, which is due to the absorption peak of N3 molecule. After assembling of Ag NPs on the TiO2/N3, there is no obvious change about the TiO2 band−band transition as compared to TiO2/N3. However, the further remarkable enhancement of absorption from the TiO2 surface state indicates that the introduction of Ag NPs enhances the optical absorption, which can forecast the selective enhancement of SERS signals in TiO2/N3/Ag attributed to the assembled Ag NPs. Furthermore, a new broad absorption in the 410−800 nm range whose maximum absorption is located around 460 nm comes out in the optical spectrum of TiO2/ N3/Ag, which is caused by the LSPR effect from the introduction of Ag.27 SERS Spectra of N3 in Assemblies and Enhanced Mechanism Analysis. In order to explore the structure of the chemisorption on the surfaces of TiO2/N3 and TiO2/N3/Ag assemblies, we compared the Raman spectrum of N3 powders and SERS spectra of TiO2/N3 and TiO2/N3/Ag assemblies in Figure 3A. The assignment of the main bands of N3 in different assemblies was reported in a previous publication.8 The band assignments are summarized in Table S1 provided in Supporting Information. Compared with the free N3 spectrum, a new band associated with νs(COO−) appears at 1366 cm−1 after the dye adsorption, which indicates that the N3 molecule anchored to TiO2 with the carboxylic acid groups from the bipyridine (bpy). The intensity of the band at 1715 cm−1 associated with ν(CO) decreases largely in TiO2/N3 assembly. The ν(CO) still exists, meaning that not all the COOH groups participate in the chemical adsorption. This conclusion is in accordance with the previous report that N3 anchors onto the TiO2 surface with half carboxyl groups from the trans isomerism of the bpy to the isothiocyanate groups.5 Meanwhile, the ν(CN) from the SCN group at 2067 cm−1 clearly shifts to 2090 cm−1, as depicted in Figure 3B. It can be concluded that a few of the SCN groups also participate in the adsorption with TiO2 NPs with most of the SCN groups remaining free. On the basis of the above-mentioned analysis, two adsorption configurations of N3 on the TiO2 NPs are illustrated in Figure 4. Briefly, most of N3 is anchored to TiO2 via two COO− groups with some of the COOH groups remaining free as illustrated in Figure 4A. Meanwhile, a few SCN groups are also involved in the

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RESULTS AND DISCUSSION Formation and Characterization of TiO2/N3 and TiO2/ N3/Ag. Structures of Assemblies. In this part, we utilized anatase TiO2 NPs calcinated at 450 °C to fabricate TiO2/N3 and TiO2/N3/Ag sandwich-structure assemblies. Figure 1 shows representative TEM images of pure TiO2 NPs, TiO2/ N3, and TiO2/N3/Ag assemblies. The average size of the TiO2 NPs is 7.47 nm with a standard deviation of 1.01 based on Figure 1E. As shown in Figure 1B, there is no obvious change after the adsorption of N3 molecules onto the TiO2 NPs as compared pure TiO2 NPs. It can be seen from Figure 1C that the Ag NPs are assembled on TiO2/N3 successfully, and the diameter of the Ag colloid NPs was found to be about 40 nm. Figure 1D shows the SEM image and the size distribution of the Ag NPs assembled on a glass slide. The average size of the Ag NPs is 43.23 nm with a standard deviation is 4.94 based on Figure 1D and Figure 1F. Measurements of UV−Visible DRS. UV−vis DRS are shown in Figure 2. Below 410 nm, the TiO2 NPs (A), TiO2/N3 (B), and TiO2/N3/Ag (C) assemblies show a wide optical absorption corresponding to the band−band transition of TiO2 with 3.2 eV band gap energy. There is a tail associated with the surface state of TiO2 NPs in the 410−490 nm region. By comparison of the TiO2/N3 with the pure TiO2 NPs, the C


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Figure 3. (A) (a) Raman spectrum of N3 powders and SERS spectra of (b) TiO2/N3 and (c) TiO2/N3/Ag assemblies with the excitation at 633 nm. A Raman band due to the bipyridine ring breathing at 1024 cm−1 was used to calibrate the spectrometer. (B) Enlargement of the 2000−2200 cm−1 region of the spectra shown in (A).

Figure 4. Schematic illustration of two adsorption configurations of N3 adsorbed on TiO2 NPs.

Figure 5. Schematic illustration of two adsorption configurations of N3 in TiO2/N3/Ag assembly.

D


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Figure 6. SERS spectra of TiO2/N3 (A) and TiO2/N3/Ag (B) with excitation wavelengths of 476.5, 514.5, 532, 633, and 785 nm, normalized at 1024 cm−1. Degree of CT of TiO2/N3 and TiO2/N3/Ag is plotted as a function of excitation wavelength in (C).

Figure 4B only one free SCN group remains in the configuration of N3 in the TiO2/N3 assembly to anchor with Ag NPs as illustrated in Figure 5B. Meanwhile, the bond that has formed in the TiO2/N3 assembly still exists. In these two adsorption configurations, the free part of COO− groups also participates in the adsorption with Ag NPs. It is worth noting that the band at 703 cm−1 associated with δ(ring) in-plane ring deformation is enhanced selectively. According the SERS dipole selection rules, this selective enhancement of δ (ring) verifies a perpendicular orientation of TiO2/N3 on Ag NPs. The bpy ring breathing mode shifts from 1037 to 1026 cm−1. The ν(CC) and ν(CN) of bpy ring shift from 1437, 1473, 1537, and 1603 cm−1 to 1429, 1473, 1539, and 1608 cm−1, respectively; meanwhile the band at 1539 cm−1 becomes broader. Meanwhile, the Raman spectra of TiO2 NPs, TiO2/N3, and TiO2/N3/Ag are shown in Figure S1. It is worth noting that the Raman intensity related to N3 in TiO2/ N3/Ag is about 30-fold compared to the Raman intensity in TiO2/N3. Parts A and B of Figure 6 show SERS spectra of TiO2/N3 and TiO2/N3/Ag with the excitation wavelengths of 476.5, 514.5, 532, 633, and 785 nm, respectively. In order to observe the variation of relative intensity, the Raman spectra are normalized at 1024 cm−1 (ring breathing mode). For quantitatively estimating the CM effect for SERS intensity in the TiO2/N3 and TiO2/N3/Ag assemblies, we employed the concept of the ρCT, proposed by Lombardi.22,23 For a k-bond, the ρCT(k) is described by the following equation:

adsorption with most of the SCN groups remaining free (Figure 4B). It is noted that the vibrations of the polypyridine ligands also reveal some differences in the Raman spectra between the N3 and TiO2/N3. After the adsorption of N3, the peak at 1024 cm−1 due to the bpy ring breathing mode shifts to 1032 cm−1. Meanwhile, the intensities of bpy ring stretch modes (ν(CC) and ν(CN)) located at 1437, 1473, 1537, and 1603 cm−1 also decrease and shift to 1441, 1477, 1547, and 1610 cm−1, respectively. On the other hand, the intensities of ν(CN) of bpy at 1437 cm−1 increase. After the adsorption of Ag NPs on the TiO2/N3 assembly, the Raman spectra reveal some differences between the N3 and TiO2/N3. Compared with the TiO2/N3 assembly, the peak of ν(CN) (SCN) at 2090 cm−1 splits into two nonequivalent peaks: a strong peak at 2143 cm−1 and a weak peak at 2090 cm−1, indicating the presence of nonequivalent thiocyanate groups (Figure 3B). The strong peak at 2143 cm−1 means that Ag NPs can chemically adsorb on TiO2/N3 assembly via the free part of SCN groups in the TiO2/N3 assembly. The weak peak at 2090 cm−1 suggests that the bonds that have been formed between SCN groups and TiO2 still exist. The former peak comes from the SCN groups that directly connect with Ag NPs, showing a strong peak at 2143 cm−1. The latter comes from the SCN groups far from Ag NPs, which yields a weak peak at 2090 cm−1. The intensity of band at 1366 cm−1 associated with νs(COO−) increases obviously in the TiO2/ N3/Ag assembly, which is ascribed to the increasing degree of deprotonation. Thus, Ag NPs can adsorb on the TiO2/N3 assembly through its free COO− groups. On the basis of the above-mentioned analysis, two adsorption configurations of N3 in the TiO2/N3/Ag assembly are provided in Figure 5. Briefly, in Figure 4A there are two free SCN groups in the configuration of N3 in the TiO2/N3 assembly, which can anchor to Ag NPs as illustrated in Figure 5A. However, in

ρCT (k) =

I k(CT) − I k(SPR) I k(CT) + I 0(SPR)

(1)

k

Herein, I (CT) is the Raman intensity enhanced by CT in the spectral region. Moreover, the other two lines enhanced by SPR are selected as references. I0(SPR) is the intensity from symmetric vibration mode. If Line k is a symmetric vibration E


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The Journal of Physical Chemistry C mode, Ik(SPR) = I0(SPR). If line k is a nonsymmetric vibration mode, Ik(SPR) is the Raman intensity only enhanced by SPR in the spectral region. Normally, when I0(SPR) is large, Ik(SPR) approaches zero. In this system, the band of ν(CN) and ν(C−C) at 1270 cm−1 is quite sensitive to the CT effect28 while the band at 1024 cm−1 associated with ring breathing is less so. Thus, the band at 1270 cm−1 was selected as Ik(CT) and the band at 1024 cm−1 was selected as I0(SPR). It is noted that the property of selective enhancement found in systems of N719 also appears in assemblies of N3. Thus, the eq 1 can be approximately expressed as follows: ρCT (k) =

1

I1270 I1024 I + I1270 1024

ISERS = IRRICT(N3 − TiO2)

(3)

Owing to the limitation of laser energy, the SERS enhancement originating from CT (N3-TiO2) and resonance process do not appear in any situation. The detailed contributions to SERS excited by laser wavelengths of 476.5, 514.5, 532, 633, and 785 nm are shown in Table 1. As the laser Table 1. Contribution to SERS under the Excitation Wavelengths of 476.5, 514.5, 532, 633, and 785 nm contribution to SERS

(2)

Figure 6C plots ρCT as a function of excitation wavelength. In the TiO2/N3 assembly, the numerical values of ρCT are similar under low excitation energy. When the laser energy exceeds the CT energy threshold 2.071 eV, the ρCT increases with the increase in the laser energy, indicating that the band associated with CT the process of N3 was selectively enhanced. After the adsorption of Ag on TiO2/N3 assembly, it is noted that except for the 785 nm laser excitation the ρCT values increase significantly with the same excitation wavelength. When the laser energy exceeds the CT energy threshold 1.877 eV, ρCT shows an increase with increasing laser energy until asymptotic behavior appears under higher excitation energy. This indicates a saturation effect of the SERS of mode related to the CT process. Rajh et al.29 have reported the similar asymptotic behavior. In order to explain the variation tendency of ρCT and the shift of CT energy threshold, we propose two models in Figure 7.

laser excitation wavelength, nm

laser excitation energy, eV

476.5

2.60

514.5

2.42

532

2.33

633 785

1.96 1.58

TiO2/N719

TiO2/N719/Ag

RR, CT(N3‑TiO2) RR, CT(N3‑TiO2) RR, CT(N3‑TiO2)

EM, RR, CT(N3−TiO2), CT(Ag−N3) EM, RR, CT(N3−TiO2), CT(Ag−N3) EM, RR, CT(N3−TiO2), CT(Ag−N3) EM, CT(Ag−N3) EM

energy is higher than 2.01 eV (618 nm), a photoexcited electron can transfer from N3 to TiO2, which is in accordance with the variation tendency of ρCT in Figure 6C (TiO2/N3). And the result of CT energy threshold of TiO2/N3 in Figure 6C is close to the model proposed in Figure 7A. After the introduction of Ag to TiO2/N3 assembly, the photoexcited electrons redistribute among the interface of them. As schematically illustrated in Figure 7B, the CT (N3− TiO2) and resonance effect on the whole SERS spectrum still exist. Additionally, two new parts of contribution appear: (1) the EM effect from Ag, whose intensity is shown as IEM and (2) the CT effect (electrons transfer from the HOMO of N3 (−6.01 eV) to the Fermi level of Ag (−4.26 eV)), whose intensity is shown as ICT(Ag−N3). Therefore, SERS intensity of the TiO2/N3/Ag assembly can be shown as the following equation: ISERS = IEMIRRICT(N3 − TiO2)ICT(Ag − N3) (4) The contributions of CT (N3−TiO2) and CT (Ag−N3) process do not exist in all situations due to the limit of laser energy as mentioned above. As the laser energy is higher than 1.75 eV (710 nm), photoexcited electron can transfer from N3 to Ag. Thus, the ρCT increases obviously at the same excitation wavelength. Meanwhile the ρCT shows a clear increasing trend with increasing laser energy until asymptotic behavior appears under the higher excitation energy. And the result of CT energy threshold of TiO2/N3/Ag in Figure 6C is close to the model proposed in Figure 7B. Formation and Characterization TiO2/N3/Ag Assemblies Based on Different Crystalline Phase Structure TiO2 NPs. In order to study the influence of crystal structure of TiO2 NPs on SERS in the TiO2/N3/Ag assembly, we utilized TiO2 NPs calcinated at 400, 450, and 500 °C as substrates to fabricate TiO2/N3/Ag assemblies. The XRD patterns of TiO2 NPs calcined at 400 °C, 450 °C, and 500 °C separately are shown in Figure 8. The XRD peaks located at 2θ = 25.4° (101) and 2θ = 27.4° (110) come from the anatase and rutile phase of TiO2.32 TiO2 NPs calcinated at 400 and 450 °C exhibit anatase phase. As the calcinated temperature is raised to 500 °C, a small amount of rutile phase appears. It can be concluded from the increasing intensity and the narrowed width of XRD peaks that

Figure 7. Illustrations of CT mechanism of N3 in (A) TiO2/N3 and (B) TiO2/N3/Ag assemblies.

According to a previous report, the valence band (VB) and conduction band (CB) of TiO2 are at −7.20 and −4.00 eV, respectively.30 The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of N3 are located at −6.01 and −3.65 eV.31 The Fermi level of Ag is located at −4.26 eV referenced to the vacuum level. In the TiO2/N3 assembly, because tbe N3 molecule absorbs lights with different laser wavelengths, various CT processes come out resulting in the variation of SERS signals. As illustrated schematically in Figure 7A, SERS signal of this system comes from the following two parts: (1) the resonant electron transfer between the HOMO (−6.01 eV) and the LUMO (−3.65 eV) of N3, whose intensity is shown as IRR, (2) the CT effect (electrons transfer from HOMO of N3 (−6.01 eV) to CB of TiO2 (−4.00 eV)), whose intensity is shown as ICT(N3−TiO2). Therefore, SERS intensity of the TiO2/ N3 assembly can be shown as the following equation: F


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°C), and 1.877 (450 °C), respectively. This can be ascribed to the fact that the crystalline degree of anatase TiO2 NPs calcinated under 450 °C appears relative higher and the surface defects of it are relatively more abundant. According to the XRD measurements, as the calcinated temperature of TiO2 NPs is raised to 500 °C, a small amount of rutile phase leads to a decrease of the surface defect and the specific surface area of TiO 2 NPs, which is due to the excessive increasing crystallinity.33 Thus, ρCT obviously declines at the same excitation wavelength; meanwhile, CT energy threshold shifts to 1.975 eV. The greater is the adsorption quantity of the N3 dye, the greater is the specific surface area of TiO2 NPs, and the CT process in the TiO2/N3/Ag assemblies comes from the enough surface defects. Thus, the TiO2/N3/Ag assembly based on the TiO2 NPs calcinated at 450 °C with the highest ρCT and lowest CT energy threshold is more suitable for DSSCs.

Figure 8. XRD patterns of TiO2 NPs calcined at 400 °C, 450 °C, and 500 °C.

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the crystallinity degree and diameter of TiO2 NPs increase with rising calcination temperature from 400 to 500 °C. The crystalline diameters (D) of TiO2 calcinated 400 °C, 450 °C, and 500 °C were about 6.49, 7.13, and 10.36 nm, respectively, which were estimated by utilizing the half peak width (β) of XRD peak according to the Scherrer formula:32 D = k λ/ (β cos θ). Parts A, B, and C of Figure 9 show SERS spectra of the TiO2/N3/Ag assemblies based on TiO2 NPs calcinated at different temperatures with the excitation wavelengths of 476.5, 514.5, 532, 633, and 785 nm, respectively. In order to observe the variation of relative intensity, Raman spectra are normalized at 1024 cm−1. Figure 9D shows a plot of ρCT as a function of excitation wavelength. It can be seen here that, at the same excitation wavelength, the TiO2/N3/Ag assembly based on TiO2 NPs calcinated at 450 °C exhibit a higher ρCT value as compared to that calcinated at 400 °C. The ρCT variation trend of them is similar, whose CT energy thresholds are 1.880 (400

CONCLUSIONS In this work, we chose TiO2 as the basic substrate to fabricate semiconductor−dye molecule−metal systems, TiO2/N3 and TiO2/N3/Ag. It is found that the property of selective enhancement found in systems of N719 also appears in the assemblies of N3. Meanwhile, we get the plots of ρCT as a function of excitation wavelength of them, which contribute to the understanding of the CT process in the series of N3 assemblies. In the TiO2/N3 assembly, the numerical values of ρCT are similar under low excitation energy. When the laser energy exceeds the CT energy threshold 2.071 eV, ρCT shows an obvious increasing trend with increasing laser energy. After the adsorption of Ag on the TiO2/N3 assembly, it is noted that except for the 785 nm laser excitation ρCT values increase obviously at the same excitation wavelength. When the laser energy exceeds the CT energy threshold 1.877 eV, ρCT

Figure 9. SERS spectra of TiO2/N3/Ag assemblies based on TiO2 NPs calcinated at 400 °C (A), 450 °C (B), and 500 °C (C) with the excitation wavelengths of 476.5, 514.5, 532, 633, and 785 nm, normalized at 1024 cm−1. (D) Degree of CT of the TiO2/N3/Ag assemblies based on TiO2 NPs calcinated at different temperatures as a function of excitation wavelengths. G


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increases with the increase in the laser energy until asymptotic behavior appears under higher laser energy. Furthermore, we proposed two models about the energy level scheme of TiO2/ N3 and TiO2/N3/Ag to explain the variation tendency of ρCT and the shift of CT energy threshold. Then, we investigated the influence of crystal structure of TiO2 NPs on CT process by the fabrication TiO2/N3/Ag assemblies based on TiO2 NPs calcinated at 400, 450, and 500 °C. The plots of ρCT as a function of excitation wavelength of TiO2/N3/Ag assemblies based on TiO2 NPs showed that, at the same excitation wavelength, the descending order of ρCT value of TiO2/N3/Ag assemblies based on TiO2 NPs calcinated at different temperatures is 450 °C > 400 °C > 500 °C. And the CT energy thresholds are 1.880 eV (400 °C), 1877 eV (450 °C), and 1.975 eV (500 °C), respectively. Thus, the TiO2/N3/Ag assembly based on TiO2 NPs calcinated at 450 °C with highest ρCT and lowest CT energy threshold is the most in favor of CT process. More extensive study on the difference in the CT processes among different dye molecules at the semiconductor−molecule−metal interface is now in progress in our research group.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00153. Assignments of major bands for TiO2, TiO2/N3, TiO2/ N3/Ag, and N3; Raman spectra of TiO2 NPs, TiO2/N3, and TiO2/N3/Ag (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-431-85168473. Fax: +86-431-85193421. ORCID

Bing Zhao: 0000-0002-9559-589X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Natural Science Foundation (Grants 21273091, 21221063, 21327803, 21403082, 21411140235) of the People’s Republic of China is acknowledged.

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REFERENCES

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Ag Interface Monitored by Surface

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