Ag Revealed by Surface

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Interfacial Charge-Transfer in TiO/PTCA/Ag Revealed by Surface-Enhanced Raman Spectroscopy Hao Ma, He Wang, Peng Li, Xiaolei Wang, Xiaoxia Han, Chengyan He, and Bing Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04648 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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The Journal of Physical Chemistry

Interfacial Charge-Transfer in TiO2/PTCA/Ag revealed by Surface-Enhanced Raman Spectroscopy Hao Ma,† He Wang,† Peng Li,† Xiaolei Wang,† Xiaoxia Han,† Chengyan He,‡ Bing Zhao*† † State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China ‡

China-Japan Union Hospital of Jilin University, Changchun 130033, P. R. China

Abstract: Charge-transfer across the interface of TiO2, 3,4,9,10-perylene tetracarboxylic acid (PTCA) molecules and Ag with three models (TiO2/PTCA, TiO2/PTCA/Ag, Ag/PTCA/TiO2) has been investigated using surface-enhanced Raman scattering (SERS). It is found that charge transfer from PTCA molecules to TiO2 takes place in TiO2-PTCA interface. After compositing with Ag, we find that the SERS spectra of TiO2-PTCA-Ag shows an unexpected dramatic enhancement at 785 nm, and reveals an unusual wavelength-dependence. On the basis of our results, we propose a new pathway of charge seperation, which is induced by not only a charge-transfer resonance but also a D-π-A charge transfer mode. We demonstrate this mechanism in TiO2/PTCA/Ag system, in which the silver electron can be transferred to TiO2 through two CT processes in series. Thus, it is a typical example of CT system to enrich the chemical mechanism of SERS, which also provides a new advance toward a better understanding of the interface between dye and TiO2.

Introduction In model studies, the interfacial charge-transfer of organic dye and semiconductors has attracted considerable attention as one of candidates for photovoltaics and photocatalysis.1-4 As is well known, this ultrafast charge-transfer process usually occurs from photoexcited state of dye sensitizer into semiconductors with a large band gap, mostly TiO2. Perylene derivatives such as 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), have been widely used in organic devices and also serves as model organic molecules for research.5-7 Therefore, the interest of many research groups is focusing on interpreting the interfaces between perylene derivatives and semiconductors for a better understanding. To this end, several methods such as core-hole clock spectroscopy,8 differential reflectance spectroscopy (DRS),9 X-ray photo-emission spectroscopy10 and so forth11,12 have been successfully applied to investigate the charge-transfer process of PTCDA/semiconductor systems. Surface enhanced Raman spectroscopy (SERS) is a promising technique for chemical sensing,13,14 biological analysis,15,16 and also studying CT process.17-19 It is known to all that electromagnetic enhancement (EM) is generated by the surface plasmon resonance in metallic particles, which generally contributes most of SERS

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intensity. In the case of semiconductor materials, both conduction band (CB) and valence band (VB) plasmons are rarely contributed to SERS in the visible region.20 Accordingly, their enhancement involves a charge-transfer mechanism. The CT mechanism, as well as in metal system, is also a resonance Raman-like process but decided by excitation energy and CT directions.21 To interpret the SERS intensity in molecule-semiconductor system, Lombardi and Brike employ a similar Herzberg-Teller (HT) selection rules as in metal:22,23 Γ(Qk)= Γ(µCT)⊥×Γ(µmol) (1) ⊥ Γ(Qk)= Γ(µCT) ×Γ(µex) (2) For molecule-semiconductor systems, there are two kinds of CT transitions: a, molecule to semiconductor (HOMO to CB); b, semiconductor to molecule (VB to LUMO), corresponding to equation (1) and (2) respectively. Note that Γ(Qk) refers to irreducible representation of allowed vibrational transition; Γ(µCT) is the irreducible representation of CT transition; and intensity of molecule or exciton transition are borrowed from Γ(µmol) or Γ(µex). It provides apporach for interpreting CT enhancement, which paves the way to explore CT interfaces by SERS. Although Raman and SERS of PTCA and its diimide derivatives have been widely reported,24-26 the semiconductor enhanced Raman spectra of PTCA is still lacking. In this work, a series of assemblies with PTCA, TiO2 and Ag have been fabricated for the first time. Based on HT selection rules, we show that a molecule-to-semiconductor occurred in TiO2-PTCA interface. The results are totally consistent with vibronic mechanism of CT of our previous work. By introducing Ag and degree of CT as a measure, the SERS signals of PTCA exhibit obvious difference in Raman spectra and a higher CT degree which is attributed to D-π-A CT mode. Moreover, we also find the assemblies of TiO2/PTCA/Ag involves a charge-transfer induced charge seperation. A possible plasmonic mechanism that LSPR can shift to NIR region through charge transfer process is developed, which is different from the previous mechanisms in metal-semiconductor heterojunctions.

Experimental section Materials. Commercial TiO2 is rutile titanium (IV) purchased from Sanbang Reagent. 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), Sodium citrate, AgNO3, HAuCl4, KOH and poly(diallyldimethylammonium chloride) (Mw = 200,000−350,000, 20 wt% aqueous solution) were obtained from Sigma-Aldrich.

Instrument SERS and normal Raman spectra were measured on Renishaw Raman system model 1000 spectrometer and a Jobin Yvon/HORIBA LabRam ARAMIS Raman spectrometer with 532, 632.8 and 785 nm laser. The typical exposure time for SERS measurement applied in this work was 1 s with one-time accumulation, and for normal Raman was 5 s with two-time accumulation with a 0.3D filter. The ultraviolet-visible (UV-vis) spectra of the samples were obtained with a SHIMADTU


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ultraviolet spectrophotometer (UV-3600). UV spectra of powder samples were obtained using a HITACHI spectrophotometer(U-4100).

Preparation of Assemblies. In this work, we utilized two kinds of substrates: TiO2/PTCA & TiO2/PTCA/Ag and Ag/PTCA & Ag/PTCA/TiO2 chips. TiO2/PTCA & TiO2/PTCA/Ag: PTCA aqueous was obtained by hydrolyzation of PTCDA in aqueous KOH. 0.3 g TiO2 NPs was dissolved in 15mL PTCA aqueous solution (10-5 M), the mixture was stirred overnight at room temperature. Then, the precipitate was centrifuged at 7000 r/s and rinsed with distilled water five times to remove unbound PTCA. For obtaining TiO2/PTCA/Ag assemblies, 5 mL of Ag colloid was added into the above 5mL TiO2/PTCA assembly; the mixture was stirred for 5 h. Finally, the precipitates were centrifuged at 7000 r/s, rinsed with water for five times, and dried under vacuum at 60 ℃ to be measured. The whole assembly is schematically shown in Figure 1a. Ag/PTCA & Ag/PTCA/TiO2 chips: This sandwich system was fabricated by self-assemble process, which has been reported in our previous work.27 For convenience, we provided the detailed information in the supporting information. Also, the sandwich system is described distinctly in Figure 1b.

Figure 1. Illustration of different assemblies. (a) TiO2/PTCA/Ag and (b) Ag/PTCA/TiO2.

Result and discussion Formation and Characterization of TiO2/PTCA/Ag. Figure 2a shows the TEM images of TiO2 NPs. It can be seen from Figure 2a that the average size of TiO2 NPs is 20 nm with rutile crystal phase, which is confirmed by XRD (shown in the Figure S1). And no remarkable change is observed after adsorption of PTCA molecule, which is also consistent with XRD result. Actually, we have tried to use TiO2 with different diameter. However, only on a large diameter can we obtain a good adsorption of PTCA which may due to its large crystal surface. Also, in the present TiO2/PTCA/Ag system, we can observe several AgNPs composited with TiO2 in the vision (shown in Figure 2b), which indicates that TiO2 dominates in this system. To confirm the composition of TiO2/PTCA/Ag, we utilized XPS as a tool. The



XPS results (shown in Figure S2) indicates that TiO2/PTCA/Ag has been fabricated successfully.

Figure 2. (a) TEM of TiO2 NP. (b) TEM of TiO2/PTCA/Ag assemblies. (c) UV-vis spectra of TiO2, TiO2/PTCA, TiO2/PTCA/Ag, and TiO2/Ag, respectively. The scale bar of a,b are 100 nm. In the case of UV-Vis spectra (Figure 2b), the absorption of pure TiO2 exhibits a wide absorption below 350 nm which is attributed to the band-band transition of TiO2. According to spectra, we can calculate its band gap of 3.25eV (Figure S3). Comparing TiO2/PTCA with pure TiO2, a big absorption emerges at 480 nm which can be attributed to absorption of PTCA. To our surprised, after compositing with Ag, we observe a slight red shift and a broaden new band. These results indicate a formation of CT complex where we deduce a CT would occur. The involvement of CT induced charge separation can be confirmed. SERS spectra of assemblies. We perform a wavelength-dependence study of different assemblies, as can be seen in Figure 3. Firstly, we investigated the SERS spectra of TiO2/PTCA at different wavelengths. In high-frequency region, a number of bands appeared at 1293, 1588 cm-1, which correspond to ring breath vibrations with A1 symmetry.28 And photon vibrational modes of TiO2 (A1g, B1g, Eg) dominated the low-frequency region.29 It is worth noting that 532 nm and 633 nm revealed the same spectra. When the laser lower to 785 nm, a new band emerged at 537 cm-1, which was assigned to non-totally-symmetric vibrations of b1 symmetry.30 It is deduced that this enhancement should involve a molecule to semiconductor CT process. Secondly, after composition with Ag, we observed a wavelength-dependent spectra more clearly. In


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SERS study, noble metals such as silver always serve as an enhancer. As shown in Figure 3, despite of 532 nm, the SERS signals were enhanced dramatically under laser excitation of 633 and 785 nm. It is noted that CT from Ag to molecule contributes to the peak of b1 at excitation of 633 nm. A strong enhancement of non-totally-symmetric vibrations of b1 was observed at 537cm-1, 793 cm-1 under 785 nm, and we found that both metal-to-molecule CT and molecule-to-semiconductor CT contributed the enhancement in the TiO2/PTCA/Ag at excitation of 785 nm, which can be confirmed by multiple peaks of b1 mode at 537cm-1.

Figure 3. Wavelength-dependent TiO2/PTCA/Ag.

SERS

spectra

of

TiO2/PTCA

and

However, this result we obtained at 785 nm is inconsistent with our prediction based on previous work31 but aligned with recent result of Chiang et al.32 As a matter of fact, the adsorption orientation of PTCA changed on the TiO2, which is lying flat on the TiO2 surface rather than to be perpendicular on metal surface. The PTCA prefers to adsorb the TiO2 surface via its four carboxyl to keep its lowest energy in TiO2/PTCA system. While in the Ag/PTCA system, stable adsorption of the PTCA occurs when only two carboxyl groups adsorb on the Ag surface, resulting in the difference in the orientations between two assemblies. This is also to say, the x-axis and y-axis of PTCA are parallel to TiO2 surface with symmetry lowering to C2v, and the charge-transfer dipole moment operator should be A2 in C2v. In our previous work,31 we have calculated allowed excited-state of PTCA (B3u), and it should be lowered to B2 in C2v.33 According to HT selection rules (eq 1), ⊥ Γ(Qk)= Γ(µCT) ×Γ(µmol)= B2xA2=b1 Thus, the charge-transfer state must be of symmetry B1 which in turn allow b1 symmetry vibrations as observed in TiO2/PTCA. Likewise, given that the CT dipole moment of Ag is same as that on the TiO2, we confirm that the CT vibronic mode from Ag also belongs to b1 (B2xA2) symmetry. Thus, the theory matches the observations well. Although we interpreted the CT direction based on HT selection rules successfully,



these results are not enough to elucidate the mechanism of CT. Hence, we analyzed the SERS wavelength-dependent spectra of Ag/PTCA/TiO2. We found that the spectra of Ag/PTCA/TiO2 were present like in TiO2/PTCA/Ag when soaked in 4 hours (data not shown). This result indicates that the orientation of PTCA is attached on the silver with its x-axis and y-axis parallel to the surface which is in agreement with result above. Similarly, the Ag/PTCA/TiO2 soaked overnight revealed an enhancement of b1 at 550 cm-1 which is also involved a CT process (Figure 4a). In this case, PTCA is perpendicular to the silver surface with the long z-axis through weak Ag-O bonds, which leads to symmetry lowering to C2v.31,33 In Figure 4b, comparing with spectra of Ag/PTCA, it is clear that there is a tiny peak emerged after adding TiO2 which can be ascribed to b1 vibration mode of ring deformation. Note that TiO2 layer is covalently bonding to PTCA and parallel to the silver surface (Figure 1b), which means its charge-transfer dipole moment operator is still A1. Based on our previous work, the enhancement of b1 (A1xB1) is emerged as expected.

Figure 4. Wavelength-dependent SERS spectra: (a) Ag/PTCA, (b) Ag/PTCA/TiO2. All spectra in (a), (b) were obtained on the same silver substrates, respectively. Mechanism of Charge Transfer As mentioned above, we demonstrate the results of UV-Vis spectra are ascribed to CT induced charge separation. In metal-semiconductor heterojunctions, there are two mechanisms for charge separation in the near-field: (i) direct electron transfer (DET),34 (ii) plasmon-induced resonance energy transfer (PIRET).35 In the case of DET or PIRET, the plasmon should interact with the semiconductor, then the plasmon


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can induce charge separation with incident light.36 In this system, the surface of TiO2 was saturated with PTCA before compositing with Ag. It is worth noting that the DET process also occurs in semiconductor-molecule-metal where electrons with high energy to overcome the Schottky barrier.37 However, unlike DET, the SERS spectra shows enhancement of two peaks which indicates that there are two kinds of CT processes in TiO2/PTCA/Ag. On the other hand, it is very interesting that the UV-Vis results fits the theories of PIRET well. PIRET is dependent on the absorption overlap and distance.35 In TiO2/PTCA/Ag, the absorption of TiO2 and Ag has an overlap before 400 nm. As shown in Figure 2c, the LSPR shift to NIR region, which is consistent with previous work.38 However, it is known to all that PIRET occurs in semiconductor-metal. To verify the effect of PIRET of TiO2/Ag, we prepare the composition of TiO2/Ag and test its UV-Vis spectra. As shown in Figure 2c (brown line), it is obvious that PIRET effect exists, but it should not be the dominated effect to induce charge seperation. The conclusion indicates that PTCA plays an important role in this system, which leads to a new CT pathway. Furthermore, it is to say that the SPR contributes approximately the same at different incident wavelength. To elucidate the mechanism of this system, ultraviolet photoelectron spectrometer (UPS) was selected, as it is a proven technique to explore energy level. Together with UV-vis spectra, the calculation results of Ag/PTCA/TiO2 show in Figure S4-6. The Fermi level of Ag is situated at 4.80 eV from the vacuum level. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PTCA locates at 6.01 eV and 3.35 eV, respectively. The LUMO and HOMO of TiO2 are plotted at 7.40 eV and 4.45 eV. As shown in Figure 5, it is obvious that the laser excitation at 785 nm (1.58 eV) is expected in resonance to transfer electron from molecule to semiconductor. Hence, we propose a mechanism of charge-transfer induced resonance energy transfer from Ag to TiO2. The LSPR will be shifted to NIR region through two charge transfer processes, which in turn enhance and boarden absorption. PTCA in TiO2/PTCA/Ag serves as bridge and also a gate of resonance energy transfer where only matched energy will be allowed. We now use this theory to interpret the wavelength-dependence properties on SERS intensity. Here, we introduce two hybrid factors to represent the intensity under CT resonance as follows: I= BPTCA→Semi + CAg→PTCA. B and C terms CT resonance of molecule-to-semiconductor (PTCA → Semi) and metal-to-molecule (Ag → PTCA), respectively. To simplify the expressions with consideration of HT selection rules and incident energy,22,23 we can predict the intensity using following expression:

I ∝ B + C = b ∙

+c ∙ ∙ ∙ !"

Briefly, b and c are coefficient of two factors. #$ represents the electronic transition from HOMO to LUMO. #%& represents electronic transition from HOMO to VB of semiconductor. #'( represents electronic transition from fermi level of metal to LUMO. Therefore, a resonance is expected when # = #%& )* # = #'( . As shown in Figure 5, we find # ≈ #%& ≈ #'( in this system. This is to say, two CT processes should resonant at same incident light, which induce a resonance energy transfer as a



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result.

Figure 5. Illustration of CT mechanism of Ag/PTCA/TiO2. Another important thing is, as well as in TiO2/PTCA/Ag system, the low excitation (785 nm) induced two CT processes: metal-to-molecule CT and molecule-to-semiconductor CT. Which process is dominant in this femtosecond-level time? Herein, we applied the degree of CT (,-. ) for interpreting the CT contribution of different processes more accurately.39,40 Based on our previous work, ,-. (k) of a k-band can be determined as followed equation: ,-. / =

0 1 -.01 23$ 01 -.405 23$

(3)

Note that one line of Ik(CT) is the intensity of a band which is attributed to SERS intensity by CT resonance. Also, it is important to choose reference bands where there is no CT contribution to SERS intensity, the line of Ik(CT), which is originated from SPR. Usually, Ik(SPR) is very small or zero when the CT vibrations are nontotally symmtric. In this case, we selected the ring breathing Ag (A1) mode at 1293 cm-1 as one line of I0(SPR), which is enhanced in whole spectrum. As for Ik(CT), we selected the band at 532 cm-1 and 527 cm-1 (b1 mode) for the investigation of CT of molecule-to-semiconductor (PTCA → Semi) and metal-to-molecule (Ag → PTCA) individually. Similarly, 538 cm-1 and 550 cm-1 (b1 mode) were selected in Ag/PTCA/TiO2 system as Ik(CT), respectively. Thus, eq 3 can be expressed as follows: ,-. / =

067→9 6 : 9 6→;?@ B4067→9 6 : 9 6→;?@

(4)

In this way, we can evaluate various CT resonance for a better understanding


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quantitatively. As shown in Figure 6, we calculated the value of ,-. in different systems, which is directly proportional to CT contribution.

Figure 6. Degree of CT for different systems: (a) TiO2/PTCA, (b) TiO2/PTCA/Ag, (c) Ag/PTCA, (d) Ag/PTCA/TiO2, under 532, 633, 785 nm excitation. Each data point represents an average of three measurements. Clearly, after introducing Ag, the intensity and ,-. increased from 0.3 to 0.4 under 785 nm laser excitation in TiO2/PTCA/Ag system (Figure 6a, b). As shown in Figure 6d, after layer-by-layer adsorption, the intensity of IPTCA → Semi revealed ,-. of 0.1 under 532 nm and 633 nm laser excitations, and up to 0.4 under 785 nm laser excitation. These results indicate that silver plays a very important role in this multi-charge-transfer system. It is very interesting that ,-. of Ag-to-PTCA decreased under 785 nm laser excitation. This should be attributed to a donor-π-acceptor CT model.41 We demonstrate that CT of Ag to molecule should occur first, which in turn provides a pressure from LUMO to induce an increase of CT of molecule to TiO2, and the IPTCA → Semi obtained from not only CT of molecule-to-semiconductor, but also from IAg → PTCA. Note that the IPTCA → Semi of about 0.1 under off-resonance excitation is reasonable in Ag/PTCA/TiO2 system because of intensity-borrowing from IAg → PTCA through D-π-A path, which we cannot observe in TiO2 dominating system. Taken together, we conclude that the CT of metal to molecule dominated the CT process.

Conclusion In conclusion, we first investigated the wavelength-dependent Raman spectra of TiO2/PTCA interface and observed a vibronic modes (b1) associated with CT process between PTCA and TiO2. Based on HT selection rules, we confirmed the adsorption orientation of PTCA on TiO2 surface and CT direction. Then we introduced Ag as an enhancer to further explore the mechanism of CT. To our surprise, a CT resonance induced both of metal-to-molecule and molecule-to-semiconductor charge-transfer process. Finally, with help of degree of CT ( ,-. ), we found that this multi-charge-transfer resonance involved a D-π-A CT mode. It is expected that our results will not only be helpful to enrich molecule to semiconductor CT mechanism but also be valuable for deeper understanding interfacial CT of dye/semiconductor.



ASSOCIATED CONTENT Supporting information: Detailed information of preparation of Ag/PTCA and Ag/PTCA/TiO2 system; XRD, XPS of different TiO2/PTCA/Ag assemblies; Band gap TiO2 of NPs; UPS of Ag, Ag/PTCA, Ag/PTCA/TiO2 systems; UV-vis spectra of PTCA, Ag/PTCA/TiO2 system. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Tel.: +86-431-85168473. Fax: +86-431-85193421 (B.Z.).

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation (Grant Nos. 81572082, 21327803, 21773080 and 21711540292) of P. R. China

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